Deliverability recommendations

Deliverability Recommendations

The primary goal of any email marketing campaign is to drive customer engagement and retention, while continually improving ROI. Many factors go in to implementing successful email campaigns, and this section aims to highlight as many as possible to help you deliver better results.

Email deliverability or inbox placement refers to the number of messages that were accepted by ISPs, regardless of whether the messages were placed in the inbox or junk folder. It is essentially the percentage of emails that didn’t bounce.

Deliverability remains an important factor in email campaigns to be able to drive revenue, your emails must first be delivered to your subscribers’ inboxes. If for any reason they are not being delivered, email content won’t have a chance to be viewed, clicked or drive any results for your business.

Before going into the best practices on how to improve email deliverability, it is important to first understand both the functions and the importance of spam filters.

When you hit send, your emails travel through a set of funnel filters before they reach their intended recipients. One of the critical steps of the funnel is to avoid being diverted into the spam folder.

Email Spam Filters

ISP providers like Gmail, Outlook, and Yahoo Mail use advanced spam filtering methods to protect their users and mail services from exploitation. These spam filters act as the first line of defense, analyzing incoming emails and allowing the good ones to reach the inbox while filtering the bad ones into the spam folder, or rejecting them completely.

To help you improve your email deliverability and ultimately get you more opens and clicks, follow the below guidance in these deliverability recommendations to improve deliverability performance.

Dedicated IPs

Your IP address reputation, domain, and email address all play a role in getting your email into your customers’ inboxes. Each message that bounces or that is reported as spam counts against the reputation of the originating IP. This affects email deliverability and creates issues for future messages from that IP or leads to them being blocked entirely.

Domains and IP addresses with a good reputation have a higher delivery rate therefore building and maintaining a good reputation is critical to the success of your email.

You can read more about dedicated IPs and how to obtain in the Dedicated IPs section of these docs.

IP/Domain Warmup 

Before you start using your dedicated/shared IP to send to your entire subscriber base, it’s recommended to do an IP warmup by sending emails in increments.

As a best practice, sending email campaigns using a dedicated IP requires a consistent, healthy volume. If you’re using a dedicated IP and sending inconsistent volumes, with dips and spikes, you risk being classified as a spammer.

At Infobip, we offer dedicated IPs to high-volume senders. Our dedicated team also helps with automated dedicated IP warmup by creating incremented sending batches to your recipients to help you gain trust with ISPs.

You can read more about IP/Domain Warmup and how to perform it in the web interface in the IP/Domain Warmup section of these docs.

Campaign Subdomain Separation 

If you send both transactional (such as password resets and order confirmations) and marketing emails (special promotions, marketing content) from one domain, you don’t have an efficient way of ensuring that the critical customer-specific emails are reaching inboxes, and if they are being affected by promotional emails that might be experiencing lower engagement.

Gmail recommends separating your IPs according to function to help ensure that your mail receives the best delivery possible.

A domain is unique to your website, for example, A subdomain is the child of that parent domain, identified by a prefix that indicates that it’s a distinct subsection of the larger domain.

Subdomains have a separate reputation to parent domains. This means that if someone from your company sent out a marketing campaign without the unsubscribe link and a recipient complained, your primary domain is protected from complaints, and its reputation stays intact.

Once you have decided on what your subdomain should be, you can put anything before the @ symbol in your email addresses, such as:


Authentication helps identify ownership of a mailing domain and helps protect from spammers and phishers who continue using email to distribute harmful messages.

By enabling Sender Policy Framework (SPF) and Domain Keys Identified Mail (DKIM), you’re verifying your sender reputation that proves to the ISPs that you own the domain.

Emails that are not properly authenticated are likely to have email deliverability problems and end up either in the spam box or undelivered.

Once authenticated with SPF and DKIM, you’ll need to create a DMARC record for your email to ensure your email is properly authenticating and provides warnings for authentication failures and fraudulent activities.

Sender Scores 

ISP providers like Gmail, Outlook, Yahoo Mail, use advanced spam methods to protect their users and mail services from exploitation. These spam filters act as the first line of defense, analyzing incoming emails and allowing the good ones to reach the inbox while filtering the bad ones into the spam folder (or rejecting them altogether).

The Infobip email spam filter checks your email content against its list of predefined rules and algorithms to determine if the message is spam before you can send emails via the web interface. A hit on any of the criteria is assigned a score, which is then used to calculate a combined score called an email spam score.

Google Postmaster

You are advised to create a Google Postmaster account with which to register your sender domains, and then share read access with Infobip.

This means that you can be alerted when domains exceed Gmail's spam complaint limits.

To create the Google Postmaster account:

  1. Go to (opens in a new tab) and click Get Started.
  2. Sign in with either a Gmail account or your work email address (should your work email use Google Workspace) and password.
  3. Click Get Started and add the sender domain you use from which to send emails.
  4. Copy the TXT record that Google provides. Alternatively, you can click Adding a CNAME record and copy those values, if you prefer to use CNAMEs.
  5. Log in to the DNS provider for your domain and configure this TXT or CNAME record (or you can ask your IT team to do it for you).
  6. Then, click the three dots next to your sender domain in Google Postmaster and select Verify Domain.
  7. Next, go back to the three dots next to your sender domain, and select Manage Users.
  8. Click the red "plus" button in the bottom-right corner of your screen and add a new user with email address [email protected].
  9. To add multiple sender domains, go back to (opens in a new tab), click the red "plus" button in the bottom right corner. Add your domains and then add [email protected] as user to them too.

Reputation Metrics 

ISPs have stated that they look at how many emails are opened and how many are deleted without being opened as a factor in their spam filtering decisions. This has a significant effect on your inbox placement and affects email campaigns incorrectly flagged as spam. According to ReturnPath, about 21% of permission-based emails sent by legitimate email marketers end up in junk folders.

  • Subscriber complaint rate – When your users hit the “Mark as Spam” button, ISP providers receive a notification. Be aware of your spam score, because even senders with a small number of complaints are automatically locked by mailbox providers.
  • Blocklists – Blocklists are lists of sender domains (for example and servers, or IP addresses that have been caught sending email spam. If you send an email from a blocklisted server or domain, it is likely to be directed to junk, or not arrive at all.
  • Spam traps – Identify how your subscriber data is collected across all digital touch points and ensure you have proof of all opt-in's. Remove any third party or purchased email addresses from your list.
  • Bounce rate – A bounce means your subscriber didn’t get the email you wanted them to see. This happens when an email can't be delivered to a receiving email address. The benchmark for bounces should be around 2% - anything above deserves your attention. If your bounce rate is around 5-10% on average, this means there are significant issues with your list, and you need to investigate further. Continue with this section to learn more about bounce rates.

Spam Traps 

ISPs and anti-spam groups use spam traps as a fraud management tool to find people who send to out-of-date lists or lists that they don't have permission to send in order to block emails from them.

Spam traps look like regular email addresses and are often created from old email accounts that are no longer used by their original owners. They place the address on the Internet where people or robots that harvest email addresses can illegitimately find them.

Email addresses collected this way are usually shared with other spammers or added to bulk mailing lists that are sold to people who may not understand the consequences of emailing people without their permission.

If a spam trap address ends up on one of your subscriber lists, and you send to that list, it's called "hitting a spam trap". This means you're immediately flagged and blocklisted.

You can prevent this by using email validation to verify your subscriber email list.

Bounce Rates 

Successful email campaigns depend on good quality emails. Bounce rates are an important way to tell whether users are engaging with your email campaigns. An email bounce occurs when a email message is rejected by the receiving email server, and so it is not delivered to the intended recipient.

Two types of failure status can be assigned to returned emails:

  • Bounced emails are permanent failures of delivery, such as due to an invalid email address
  • Dropped emails are temporary failures of delivery, such as due to the recipient’s inbox being full, both based on conditions with the recipient’s mail server

The bounce rate is the percentage of bounced emails out of all emails sent during a window in time. The time window may be hourly, daily, and monthly.

The formula for calculating bounce rate is as follows:

Number of bounced emails in time window

Number of emails sent with final statuses in time window

When an email is bounced, an error is sent from the receiving email server. The following error codes impact the bounce rate calculation:

  • 6012 - EC_HARD_BOUNCE

See ../../essentials/response-status-and-error-codes for more information about error codes.

When the bounce rate exceeds 3%, you first receive a warning notification.

When the bounce rate surpasses 5%, the registered sender is be disabled, which means that no more emails can be sent. You are then informed about your disabled domain, and the blocked domain is displayed in red. The notification states the:

  • bounce rate percentage
  • date of the blockage
  • time of the blockage

You also receive steps on how to improve deliverability.

After 24 hours, your domain is automatically enabled again.

While it is always possible to send bad traffic and not track it, recipient domains (ISPs) take note of all the incoming traffic. Ultimately, your bounce rate is indicative of a healthy list. The higher the bounce rate, the more ISPs identify you as a poor sender, resulting in more of your emails being diverted to the junk folder or even not being delivered to recipients at all.

All of these actions can lead to an IP blocklisting, which puts both your and Infobip resources at risk.

To avoid IP blocklisting, it is important to pay attention to any potential bounce rate blockage that might occur on your domain. In addition, you should apply good sending practices, including the following advice:

  • Only send email to contacts who have subscribed to your emails
  • Never send emails to contacts scraped from websites, third-party sources, or purchased contact lists
  • Clean your email lists on a regular basis
  • Validate your new subscribers’ email addresses
  • Respect unsubscribers, do not retarget them
  • Avoid a drastic increase in mailing volumes
  • Make sure your email content is clean


Marketers already know that crafty subject lines and well-curated content doesn’t only compel users to click through and read the emails, it also helps stand out in a crowded inbox.

Spam filters also look at the subject line, preheader, and body content. One of the aspects a spam filter looks for is whether the email is in HTML if it includes any unsupported HTML code such as scripts and forms, whether embedded images are present along with any formatting and coding errors.

The content is scanned for keywords and phrases that trigger spam filtering. You have control over what you put into your email, so make sure to run a spam check before you hit send.

Here are some of the most common causes and phrases that you should avoid when sending emails that have been proven to trigger spam filters:

  • An entire email composed of capital letters
  • Frequent, random capitalization
  • Excessive punctuation, especially "$" and "!"
  • Strange spacing or excessive amounts of blank space
  • Poor spelling
  • Frequent variations in text color and size
  • Scam-like subject lines
  • “Eliminate your debt”
  • “Risk-free”
  • “FREE!!!”

Subscriber Preferences 

One of the best ways to build trust with your subscribers is to allow them to decide which channels they prefer to be contacted on (SMS, app notifications, email etc.), which subjects they’re interested in, the types of preferred messages, and the frequency of the communication.

To comply with the law and to ensure your emails reach their recipients, it is imperative to include an unsubscribe link in your email campaigns. As the email service provider, ISPs require us to manage your lists to address:

  • Handle and remove unsubscribes from your list automatically
  • Process bounce records
  • Report abuse and other information through ISP feedback loops. ISPs closely monitor how many unsubscribes occur—if there are too many, it can lead to email deliverability issues for your account

Legal Compliance 

An effective preference center that provides a functional unsubscribe link for your email is mandatory to comply with the law. The CAN-SPAM Act requires that all unsubscribes are valid, functional and honored within ten business days.

The act also requires that you have explicit opt-in permission from your users. Confirmed opt-in lists are more engaged from the start and effectively help you to build your sender reputation.

If you are sending emails to people who weren’t aware or didn’t agree to receive messages from you, they mark your emails as spam. Consequently ISPs assume that you are spamming everyone on your sender list, even those who have opted-in to receive your emails.

Email Opt-In Laws:

Failure to comply with local laws leads to high financial penalties for non-compliance.

Email Lists 

Buying email lists is a bad practice. To use email successfully, focus on organically growing subscriber databases with email addresses of people who are genuinely interested in receiving your content.

One of the most important things you can do as part of your email deliverability check is to find out how clean and up to date your lists are. A properly acquired contact list allows you to target only the people who are likely to purchase from you.

If the recipient hasn’t engaged with you in 6 months, target them with a special message, and if that doesn’t drive engagement, remove them from the list.

By concentrating only on those who want to receive your content, you're decreasing the likelihood of bounces and abuse complaints and increasing the level of engagement and conversions.

Feedback Loops 

When one of your recipients flags your email as spam, it's considered as being non-compliant. Some ISPs provide visibility when this happens by allowing the sender to sign up for a feedback loop. This is done to aid senders with keeping their subscriber lists clean.

Senders should use feedback loops to listen to their subscribers and correct the situation by removing subscribers if there are any complaints. It’s important to learn from complaints you receive because they can help you evaluate underlying issues with your sending frequency and content.

Next Steps 

The next step is to learn more about Dedicated IPs and why they can be more beneficial than Shared IPs. Once you are done with that, or if you have already read through that section, you can learn more about IP/Domain Warmup and how Infobip uses it to make sure all of your emails are delivered.

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