The Guide to Email Deliverability

Having trouble reaching the inbox? Welcome to the ZeroBounce email deliverability guide. In this free resource, our top experts teach you everything you need to know to land your emails in the inbox.

Inboxes are crammed with emails. The average working person gets about 120 messages a day. For companies like yours, standing out in crowded inboxes is hard – that is, if you make it to the inbox in the first place. Hitting Send on your email doesn’t guarantee it.

Every day, 31 billion emails bounce. About 16% of emails go to the spam folder. You want to do everything you can so that your emails don’t end up there and reach your customers instead.

But what should I be doing?

Glad you asked.

This email deliverability guide will help you stay out of spam and get your campaigns where your audience can see them.

Our experts have decades of experience in the email industry and have helped thousands of companies land in the inbox. We put this free guide together so that you, too, can get there.

Ready to begin? Let’s start with the basics.

What is email deliverability?

Email deliverability is a term used to describe your ability to reach your subscribers' inboxes when sending email messages. Strong email deliverability guarantees that your campaigns are reaching your customers, so that you can build better email marketing ROI.

However, many senders confuse email deliverability with email delivery. They don’t mean the same thing.

Email delivery is the number of emails that manage to reach a mailbox. However, that could be in the inbox or in the spam folder.

Email deliverability refers to the number of emails that reach recipients’ inboxes. Your email service provider (ESP) will report how many of your emails were delivered, but that doesn’t mean those emails landed in the inbox.

Knowing the difference helps you better understand your email marketing metrics and the picture they paint of your overall performance.

What affects email deliverability?

Several things affect email deliverability. Your ESP plays an essential role, and so does the content and authentication of your emails. Equally important is your email score or sender reputation.

Savvy email marketers work hard to build a good sending reputation as it can make or break email marketing results.

What is your email score or sender reputation?

Your email score is a number between 0 and 100 that reflects your behavior as a sender. Much like your professional reputation, it lets Internet service providers (ISPs) know who they’re dealing with.

Are you a legitimate and trustworthy sender? Then, your emails are likely to go to the inbox folder.

Does your behavior resemble that of a spammer breaking the law and bothering people? You don’t belong in the inbox – in fact, your emails might get blocked altogether and not even get to the junk folder.

Mailbox providers handle billions of emails every day. Their goal is to create relevant user experiences, so they need a simple way to differentiate between legitimate senders and spammers. Email scores help them sort and deliver emails more effectively.

Your bounce rate has a significant impact on your email score. When more than 2% of your emails bounce, your email score takes a hit. Make sure you check your contacts before you send again.

Email deliverability guidelines: how to reach the inbox

Time to get into some email best practices to help you build a healthier email program. Use the advice below to improve your sender reputation and get past spam filters on your way to the inbox

Basic rules to boost inbox placement

Separate the types of emails you send

If your company sends transactional emails (receipts, reminders, etc) and newsletters, use a dedicated IP and domain name for each type.

For regular corporate transactional emails, use the domain “” For all other emails, use a different domain but similar to the main one such as “”

Each domain needs to send email on separate dedicated IPs. When different types of mailings are mixed, Google and other ISPs will tend to combine/group your corporate emails under the "Promotional" and "Other" tabs.

Use reCAPTCHA from Google on your registration screen

Google's reCAPTCHA 's role is to prevent automated bots from registering on your site or subscribing to your newsletters. Sending emails to bot registrations – or to people that exist but have been registered by a bot – can hurt your emailing metrics.

For example, if you email 100 bot registrations to an ISP and none of them are opened and/or clicked through, this tells the ISP that not too many people are interested in your emails. Hence, your emails are seen as junk, so your email deliverability will decrease.

Bots can sign up real people. Emailing people who didn't confirm to receive emails will hurt your inbox rate

Use double opt-in confirmation for all of your emails

This requires the user's confirmation that they’ve actively joined your email list. Double opt-in is easy to set up with your ESP. When someone registers on your site or subscribes to your newsletter, they’ll get an automated email. Your new subscriber needs to click a link in that email to confirm that they own that email address and that they want, indeed, to receive emails from you.

Use a real-time API that verifies emails on registrations and signups

Your email hygiene is essential to your inbox placement, so keep a clean email list. Validate it periodically and use an email verification API to avoid acquiring bad data.

Real-time APIs that check email addresses can tell good from bad emails apart instantly. A good email verifier will reject misspelled email addresses and fake contacts such as disposable emails.

Disposable email addresses are also known as temporary email addresses. People use them to download content offers or get a discount. These people aren’t interested in receiving emails from your company, so it’s best to block these emails from getting on your list. Temporary emails will self-destruct and bounce.

Here's an example of a place that provides disposable email addresses:

Learn more about ZeroBounce Real-Time API here: Real-Time API Documentation Real-Time API Documentation

Authenticating your emails

Use SPF authentication

Also known as "Sender Policy Framework", this is an authentication protocol that states whether an IP is authorized or not to send emails for a domain.

Here are a few places with useful resources about SPF:

Use DKIM authentication

Also known as "DomainKeys Identified Mail", this is a protocol that allows other mail servers to verify whether the email you sent has been tampered with or not. Basically, it verifies whether the email received from a specific domain was indeed authorized by the owner of that domain.

Here are a few places with useful resources about DKIM:
Here's are easy DKIM generators to help you create your DKIM Record
Here are some DKIM tools that will sign your emails. Most mail servers have this feature built in and you just need to enable it.
Here's a good DKIM testing tool:

Use DMARC authentication

Shortcut for "Domain-based Message Authentication, Reporting & Conformance", this is an authentication method that prevents phishing attacks and reduces spam. It enforces a policy that tells the ISP what to do when it receives emails, apparently from your company, that fail either SPF or DKIM.

Here are a few places with useful resources about DMARC:
Here are places you can use to Generate your Dmarc Record:

Use a reputable DNS provider

Mail delivery depends a lot on being able to retrieve records from DNS.

ISP's also check the reputation of your nameservers. Here are a few DNS providers:
  • DYN Managed DNS - Provides a lot of tools and options to secure your DNS properly.
  • Cloudflare - Provides DNS Management and a suite of optimization tools.
  • DNS from large registrars should be fine, although we have seen slowness in some registrars applying DNS changes.

Mail delivery depends a lot on being able to retrieve records from DNS.

Use a reputable CDN for static content

Also known as "Content Delivery Network", it caches versions of your content around the world.

There are four main reasons to use a CDN:
  • To provide faster load times for images included in the emails you send.
  • ISPs won't block them since they are known and reputable.
  • You will also benefit from DDOS protection which keeps your website online even if it's attacked.
  • You will benefit by default from an advanced firewall that can help mitigate some of the classic attacks looking to exploit your website.
CDN providers:

DNS Example
A Record ( > "Points To " > PTR Record > "Points to" > Hostname (
Hostname ( > "Points To" > PTR Record > "Points to" > A Record (

ISPs won't block content from CDNs since they are known and reputable.

Use PTR records (Reverse DNS Lookups) for your sending IPs

This is mandatory. Most ISPs require FCrDNS (Forward Confirmed Reverse DNS). It sounds complicated, but it's really not.

Here are a few places to test FCrDNS at:

Warming up your IPs

Warming up IPs is important. Don’t send too many emails at once. If your sending platform supports it, use limits per hour or per day for each of the IPs per domain. If you send too many emails at the beginning, you can get bounces and/or deferrals.

Here are some limits, per provider, that you should follow in the first 30 days:
  • Yahoo: 200 emails/day/IP (for at least 5 days), then you can double every day.
  • Gmail: 200 emails/day/IP (for at least 5 days), then you can double every day.
  • Hotmail: 200 emails/day/IP (for at least 5 days), then you can double every day.
  • AOL: 200 emails/day/IP (for at least 5 days), then you can double every day.
  • Cloudmark (all domains):50 emails/day/IP
  • Time Warner:100 emails/hour/IP
  • Cox: 100 emails per connection per IP, up to 5 IPs

You should always visit the postmaster website of the domains you are trying to send to in bulk. A lot of times you'll find the sending limits published within their bulk sender guidelines.

Several examples on how to warm up your IPs:
  • Start a new warm-up for the existing IP.
  • Send only to subscribers who have opened at least one of your emails in the past 30 days.
  • Limit your starting volume to 3,000 subscribers.
  • Keep sending to those subscribers only for the first three days before increasing the volume.
  • Increase the volume by 1,500 subscribers with opens in the last 30 days. Your new total volume should be 4,500 subscribers.
  • Keep sending to the new volume of 4,500 for two more days.
  • Follow this strategy to increase the volume every two or three days by 50% of the actual volume.
  • After 10 days, you can start increasing the volume by doubling it.

Feedback Loops (FBL)

Register to all feedback loops (FBLs)

Also known as complaint feedback loop, this is a service offered by some ISPs that report back complaints (when a subscriber hits the spam button) to the sender. Its purpose is to aid senders in keeping a clean list and preventing the subscriber from getting unwanted mail.

Some ISPs provide feedback loops to aid senders in keeping a clean list and preventing the subscriber from getting unwanted mail.

Feedback loops are provided by some ISPs to aid senders in keeping a clean list and preventing the subscriber from getting unwanted mail

A list on all feedback loops can be found below:

Gmail has a feedback loop that is only available for ESPs who are MAAWG members and are approved by Google as good senders.

Companies most often use the same blacklist providers. Being listed on a single blacklist can affect your delivery to many different ISPs.

Delivery monitors and IP/domain health monitors

Some of the largest ISPs provide tools to monitor your IPs, domain health and delivery.

Sign up to the places listed below and start monitoring the health of your emailing:

Google Post Master
  • Spam rate
  • IP reputation
  • Domain reputation
  • FeedbackLoop identifiers
  • Authentication (DKIM/SPF/DMARC) rates
  • Encryption (e.g. if you send emails using TLS) rates
  • Delivery Errors (for example, if you retry to fast)
  • IP reputation
  • Complaint rate per IP
  • Traps per IP and the date
  • Ehlo/Helo commands.

Email blacklisting

Make sure your domain/IPs are not on any email blacklists

Many companies use the same email blacklist providers, so being listed on a single blacklist can affect your delivery to many different ISPs.

There are a few different types of blacklists:

  • Public - These blacklists are published so any ISP can use them. They are the easiest ones to monitor using automated tools.
  • Private - These are paid email blacklists. The only way to actively monitor these is to use inbox testing tools to check your delivery.
  • Internal - These are maintained by the ISPs directly. They can be monitored with inbox testing tools, as well.

Some ISPs use multiple blacklists, so it's important to monitor all aspects of your sending using the different tools available.

Here are our favorite tools for checking if your domain/IP is on a public blacklist:
For automated blacklist monitoring, use:
How to delist in non-obvious places:
  • CenturyLink- [C10] RBL restriction: Blacklisted by Internal Reputation Service - Send an email to "postmaster [at]" with the subject line "Please investigate for spam-like qualities" and list your IPs
  • Hotmail- Fill out this form
  • United Online (Netzero/Juno/Bluelight)- Fill out this form
  • Vade Secure- Fill out this form

Whitelisting services

Whitelist your IPs after 90 days of sending

The reason you have to wait 90 days is that some whitelisting services require to see your sending history.

Whitelisting your IPs provides many benefits at different ISPs:
  • Increased inbox rates
  • Increased sending volumes
  • Less restrictive spam filtering
Free whitelisting services:
Paid whitelisting services:

Email list quality: check email addresses regularly

Try to avoid buying or renting an email list, even if it’s from a reputable provider. ESPs have ways of detecting if a list is bought or rented and they can/will reject it when you try to import it into their mailing platforms. No email validation company can help you here and the reason has nothing to do with the validity of the email recipients.

Even if you’ve built your own email list, it’s still wise to verify it every few months. Email data decays, and email lists don’t stay fresh forever. Use an email validation service to check email addresses in bulk and in real time on all your sign-up forms.

Try to get an open rate of at least 20% and a click-through of at least 1%.

We also recommend removing all non-opener emails after six months. These emails are probably valid, but not in use.

A lot of domains use sender score as a determining factor to allow emails from your IPs. If your sender score is less than 90, you should take action to improve it.

Monitor your sender score

Domains use sender score to determine whether to allow emails from certain IPs. Your sender score gives mailbox providers clear feedback on your sender reputation. If your sender score is less than 90, you should take action to improve it. You can monitor your sender score find more about senderscore

Remove risky email addresses from your list

Before you send, clean your list of bounces, spam traps and complainers. By reducing these three components from your email lists, you will increase your list quality, your sender score, your domain and IP reputation. All these factors increase your email deliverability.

We provide email validation and verification services at an excellent price.

Verify your email server is set up correctly for sending

The way your email server is configured plays an important role in your email deliverability. It pays to check your server settings before you start sending emails. There's only one way to do this accurately – use the ZeroBounce Mail Tester.

With the ZeroBounce email deliverability toolkit, you'll get a detailed technical report of your email server configuration. The report highlights any problems so you can fix them ahead of time.

Create abuse@ and postmaster@ email addresses at your sending domain

ISPs rely on these two addresses for feedback loops and complaints. Not having these two accounts set up is a red flag and they are also required by the RFC Standards.

Double-check your content

Use the recipient's name in the subject line and in the header of the HTML (e.g.: "Dear John"). Make sure you check for spam words and domains in the HTML. For example, if you link to a blacklisted domain, your email might bounce.

Inbox Test every campaign you're going to send and adjust where needed.

Industry best practices for the highest email deliverability

Remove inactive emails

We recommend removing subscribers who don’t open or click your emails after 180 days. A lot of ISP algorithms are based on user engagement. The more non-openers and non-clickers, the worse this metric gets and the more your email deliverability plummets.

After six months, you can also retry sending to these email addresses to see if the recipients open or click.

Make sure you don't use too many connections per IP

Every ISP limits the number of connections per IP. Always view the Postmaster Guidelines of the different ISP regarding Rate Limiting.

Here are some safe values to use for your connections:

Make sure you retry temporary errors

One of the most common ways that ISPs reduce spam is called "greylisting" or Temporary Errors, also known as SMTP 451. When these conditions occur, the ISP expects your mail server to re-attempt the delivery of that email at a later time.

When trying to send the email again, you must use the same IP. Rotating the IP will just get the email greylisted again.

Retry times should be configured by the schedule below:
  • First retry - 15 Minutes
  • Second retry - 45 Minutes
  • Third retry - 2 Hour
  • Forth retry - 6 Hours
  • Fifth retry - 12 Hours

This will also help if you have a configuration issue, so you won't have too many retries. If you have too many, this can affect your sender score negatively.

Monitor your brand for compliance

For larger companies, it's worth looking into using tools like Lashback that specialize in brand protection and compliance.

Use a Reply-To header that is valid

We all receive emails from noreply@, but it's better to have a functional Reply-To. You should monitor this address and respond to all customer enquiries. The more engagement between you and your customers, the better your email deliverability.

Send only one email per connection

You should only send emails to one person. Have you seen those emails with lots of "CC" or "TO" recipients? It’s a huge spam flag. When sending out bulk emails, ensure each email is addressed to a single person and not multiple contacts at once.

Enable outgoing TLS connections

Mail servers are supposed to be opportunistic in nature. What that means is they all prefer to use the most secure protocol when accepting mail - TLS 1.2, TLS 1.1, TLS 1.0, NO TLS in that order.

Some mail servers will refuse all non-TLS transmissions, so to achieve maximum email deliverability, you should always enable TLS when sending email.

Ensure that your abuse/complaint rates remain low

Once you sign up to the FBLs (Feedback Loops), as discussed earlier in this document, you need to actively remove complainers from your email list. Let go of them when you receive the alerts, so you can keep your spam complaints at bay. When these metrics get too high, they will affect your email deliverability.

Here are some guidelines to keep your abuse/complaint rates below per destination ISP:

  • Hotmail < 0.1%
  • Yahoo < 0.2%
  • AOL <= 0.3%
  • Comcast <= 0.5%

Do not use private WHOIS for your domains

All registered domains are required to have accurate information by ICANN. Hiding behind private WHOIS can hurt your domain reputation and, in some cases, it is even illegal (CAN-SPAM Act).

Have a privacy policy on your website

When you ask ISPs to whitelist your IPs/Domain, they do look for your privacy statement and verify it. If you haven’t updated it in a while, now is the time to ensure it offers complete information.


DNSSEC is a technology that was created to prevent the hijacking of DNS Lookups. This is not a requirement. However, it's a good security procedure.

Remain consistent in your sending behavior

Send your newsletters, promotional, marketing materials on the same day, every week/month. Being consistent proves you have a real business and keeps your IPs warm. The more predictable your sending behavior, the less you look like a spammer – and the better your email deliverability.

Do not segment emails per destination ISP

Separating your emails based on destination, unless under special circumstances, has a detrimental effect. Some domains will use SenderScore for reputation lookups, but those IPs will never get their sender score increased because they don't report back to Return-Path.

Do not send affiliate email marketing

Most ISPs blacklist the subject lines and content for affiliate emails. Remember, everyone is sending the same affiliate promotion. ISPs have AI that learn subject links and content. Your IPs will get blacklisted or will be limited to only being able to send to the spam folder. Normally, ISPs will accept and won’t complain if you add an advertisement to your normal newsletter.

Laws and compliance

Honor all unsubscribe requests

Keeping people on your email list against their will is not a good way to go about boosting email deliverability. When someone asks to unsubscribe – whether through an automatic link or manually – honor their request, and do it promptly.

Never wait more than 10 days to do so. This is a requirement of the CAN-SPAM law.

When you're in compliance with the law, everything becomes much easier to whitelist.

Comply with the law

Make sure you comply with CASL for CA, CAN-SPAM for US, DPEC for EU and other local anti-spam laws.

When you're in compliance with the law, everything becomes much easier to whitelist, and managing your reputation will be free of obstacles.

Here are the links to some of the major e-mail laws:

MX records

Your domain must have valid MX records

The Request for Comments (RFC) standards state that if the MX record is missing, one should use the A record as the mail server. However, we noticed that many ISPs don't follow the standard and they check the validity of your MX Record before allowing email from your domain.

If you're looking for a good mail service provider (ISP) for your domain, we recommend using G-Suite from Google.

Inbox testing

We mentioned this before and we cannot stress enough how important it is. Different ISPs use different spam filters. What might end up in the inbox at Yahoo could land in the spam folder at Gmail. Without inbox testing you can ruin your reputation and not get your emails delivered to the inbox with certain ISPs.

A useful tool for inbox testing is the ZeroBounce Inbox Tester.

Email deliverability tips to keep in mind

Many take it for granted, but as you can see, email deliverability is more complex than it seems. Implement the advice we shared so you can keep landing your campaigns in your customers’ inboxes.

Before you go, let’s do a quick recap and leave you with a few extra tips to boost your inbox placement.

  • Put permission at the core of your email marketing. Never email people who haven’t subscribed.
  • Include a visible unsubscribe link in all your email templates and honor unsubscribes right away.
  • reCAPTCHA and double opt-in are easy to set up and help you keep a more engaged email list.
  • Use a free email verifier to check every new email address you add to your list.
  • Add an email validation API to all your email gathering forms. It helps to keep bad data off your list.
  • Avoid taking ISPs by surprise with large email volumes. Warm up your IPs by sending in batches and increase volume gradually.
  • Send people the kind of content they signed up for. Your engagement is essential to your sending reputation and email deliverability.
  • Remove complainers promptly and unengaged subscribers every three to six months. Also, when checking email addresses with ZeroBounce, consider weeding out the abuse emails it finds in your list.
  • Be predictable in your sending. Show up for your subscribers regularly to keep your IP warm and your brand familiar.
  • Test your emails before you send them to see where they’re going to land. Our email deliverability tools give you lots of useful insights.
  • Sign up for an email blacklist monitor. It will alert you if your domain or IP are ever added to an email blacklist.
  • Stay away from link shorteners and strive for a healthy image vs. text balance.
  • Avoid any type of spam-like behavior and treat your audience the way you’d like to be treated.

This information was provided to you by several experts with many years of experience in email deliverability. If you’d like to contribute to this email deliverability guide, please contact us and let us know.

We also offer professional help in implementing all of the above. Reach out for details.

Frequently Asked Questions

You can improve email deliverability by practicing permission marketing, maintaining good email hygiene and sending relevant, engaging content.

Make sure you build a good sender reputation from the start and preserve it by abiding to email marketing best practices.

You can fix email deliverability by determining the potential reasons why your emails are going to spam. It could be that your email list needs cleaning, so using a free email verifier is a great start. Another reason could be that your IP or domain are on an email blacklist, so your campaigns aren’t reaching your subscribers anymore.

Also, using some reliable email deliverability tools can tell you a lot about why your emails aren’t reaching the inbox. Such tools allow you to test your inbox placement and fix any issues – before you send your email.

A good email deliverability rate is 90%. However, according to several studies, the average email deliverability rate around the world hovers around 80%.

You can ensure email deliverability by building an opt-in email list, maintaining good email hygiene and following email marketing best practices. Keeping an eye on your email score and engagement is also essential. Remove obsolete data and unengaged subscribers from your email list periodically. Also, send high-quality, relevant content and use email deliverability tools to test your campaigns before the big send.

Some of the factors that determine your email deliverability are your email score, or the way Internet service providers perceive you. A good sender reputation is a cornerstone of high email deliverability. Email with care so you can protect your reputation and help your campaigns get to the inbox.

Effective email communication starts in the inbox, and following best practices is key to achieving good email deliverability. Some of these best practices include using a valid, opt-in email list, sending relevant content regularly and avoiding spam-like behavior.

The best verification and cleaning starts with ZeroBounce

  • Get 100 free monthly email verifications