Email Deliverability – A Guide to Best Practices

Last Updated: February 17,2023

With crowded consumer and business inboxes, email deliverability and inbox placement has become a top priority for marketers in recent years. Getting emails flagged as spam or blocked by mailbox provider has an impact on sender reputation and future inbox placement.

What is Email Deliverability?

Email deliverability is the ability to deliver emails to recipients’ inboxes, while email delivery refers to ability to deliver email to mailbox provider server. Emails delivered are emails not rejected by the mailbox provider, so emails in a spam folder are considered delivered.

What Factors Affect Email Deliverability?

The major factors that affect email deliverability are:

1. Global email engagement (overtime, across campaigns)
2. Content and format of the email (spam words in subject or message body, format – use of images, etc)
3. Relative engagement for you (sender) versus other senders in the recipients inbox
4. IP reputation of the sending server
5. Sender reputation (e.g. sender being your domain or exact email address the emails are sent from)
6. Sender domain configuration and authentication (DKIM, SPF, tracking domain, etc)

Email deliverability factors

On the diagram above, the green box represents factors that vary and change more with each campaign sent and the square on the left with 4 factors are not subject to dramatic changes on a per campaign basis.

The list of best practices below can help you achieve better email deliverability from building and maintaining a strong sender reputation, while improving your engagement. 

Best Practices for Email Deliverability:

1. Maintain list quality
2. Make it easy to unsubscribe
3. Configure your sender correctly
4. Optimize sender from label and address
5. Avoid spam keywords
6. Segment and customize campaigns
7. Monitor sender reputation
8. Test your campaigns

Let’s go over this list in detail and review some helpful tools along the way. Think you have it all covered? Check out our advanced email deliverability article.

1. Maintain List Quality and High Engagement

List quality is the most common factor that influences deliverability and that many new marketers make mistakes with. Just because a list is opted-in and collected directly on the business website, it doesn’t mean “it’s all good”. Older lists can damage sender reputation severely.

Reasons emails do not get delivered

The top reasons the email does not get delivered is:

1. Email address results in a hard bounce
2. Email address results in a soft bounce
3. Email get rejected by mailbox providers

Let’s look at each of these reasons closer.

What is a hard bounce?

A hard bounce typically means an email address is no longer valid, but sadly it can be a result of the mailbox provider service being down and not responding with a correct response code that would be correctly classified as a soft bounce. All email lists decay naturally because some emails that were once active can be de-activated for inactivity (this is very common with providers like yahoo and aol) or other reasons, for example because a person leaves a job. High hard bounce rate can negatively affect future deliverability so it’s best to verify email lists that haven’t been engaged in 2+ months.

Email lists decay at an average rate of 2-3% per month, and can be higher for B2B lists.

So if you haven’t engaged with your email list for more than 6 months you are likely to have a more than a 10% bounce rate on your first bulk campaign. A bounce rate over 10% can get your account suspended with many email marketing platforms, but it also has a negative impact on your email deliverability and sender reputation, not to mention cost since you pay to send email to invalid addresses. Inbox providers pay close attention to bounce rate and may block emails from a sender that attempts to deliver to too many invalid email addresses.

What is a soft bounce?

A soft bounce means it’s temporarily not able to accept emails. There are many reasons an email can soft bounce:

1. Inbox is full (this is very common for Apple addresses like icloud.com that share storage space with other media types like music).
2. Mailbox provider service is down. Unfortunately, sometimes providers don’t respond with a soft-bounce code as they should and return a hard bounce code instead.
3. File attachment too large.

Why do emails get rejected?

Emails get rejected by mailbox providers for a variety of reasons, but usually due to sender reputation:

1. Sending volume is too large, possibly unexpected and not typical for sender’s historical volumes.
2. Sender has poor reputation or low historical engagement.
3. Sending IPs have poor reputation.
4. A combination of reasons listed above.

Unless the email list is very old and hasn’t been engaged, it’s not typical for senders with list sizes below 50000 to experience rejection issues. Rejections get recorded as soft-bounces and most platforms don’t report them separately, so the typical indication is the high % of soft bounces – when the rate is over 60% per mailbox provider.

How to avoid email rejections

First step to avoid being rejected is to identify providers and what could have caused the problem. Rejections are most common for yahoo and aol, and in rare occasions gmail, when the volume and sending speed is high.  Review your historical engagement – were the open/click rates very low?

Things you can try to increase delivery rate:

1. Throttle volume to send emails at lower speed.
2. Split larger campaigns into smaller segments to go out on separate days (still optimize for time of day though).
3. Segment to exclude inactive subscribers (no opens in the last 30-60 days).
4. Stagger sending of your segmented campaigns so you start sending to your most engaged subscribers first.
5. Check your sender domain and IP using a blacklist monitor tool and submit requests to get delisted.

How does email verification work?

In a typical verification process you upload a CSV file and once validation is complete you see stats on the results and options to download your verified list. Most verification services produce at least these 4 statuses:

1. Deliverable – these emails are safe to send to
2. Undeliverable – these addresses no longer exist and should not be sent to
3. Risky – these emails are likely to result in a high bounce rate (often catch-all addresses)
4. Unknown – these emails couldn’t be verified so may result in high bounce rate.

If your email verification provider allows it, you should download Deliverable, Risky, and Unknown as separate lists and upload them as separate lists into your email marketing platform. If you are starting fresh with a new service provider you should ONLY send your 1st campaign to Deliverable addresses only, to ensure the lowest possible bounce rate. This is because many providers move you onto a shared IP pool tier that correlates to your 1st campaign or campaigns sent in the first 1-2 days.

On average, email lists identified as Risky or Unknown by email verification providers result in 30% bounce rate, so they should be slowly added into large bulk campaigns overtime, to finalize verification through sending.

Maintain High Engagement

Mailbox providers look at past engagement to determine relevance and quality and decide if the email should go into Inbox or Spam folder. To maintain high engagement you should send less frequently to inactive subscribers on your list, and remove anyone who didn’t open your emails in over 3 months.

What’s a good sending frequency and how to decide who is inactive?

• You can send x1-2/week to those with a recent (last 2 weeks) open or recently added to your list
• Send x1/week or x2/month to those with an open within 1 month or added in the last month
• Send x1/month to inactives – anyone with 0 opens in the last 30 days and added more than 30 days ago

You need to maintain high overall engagement, so send more often to highly engaged, less often to inactives.

Improve engagement with Inbox Mailers – works on top of any ESP by signaling that your subscribers are in their inbox.

2. Make it easy to unsubscribe

Making it super easy for users to unsubscribe is not a tactic that’s obvious to benefit the sender, but it is. If email recipients can’t easily locate the unsubscribe link they are likely to hit the Spam/Complaint button instead.

Complaint is the worst type of engagement for your email campaigns. 

Your unsubscribe or “subscription preferences” link needs to allow a user to either opt-out from your brand communications or select the message types they want to receive and not to a page that requires them to log in to their account to retrieve or save their preferences. The latter has become a more prevalent practice among some high profile brands that use their own built-in-house email tools. This practice is actually in conflict with anti-spam laws. It’s advisable to provide users with a straightforward method for getting off an email list and honor that request in a timely manner.

Example: BigMailer.io unsubscribe page with preferences format

It’s best to keep the text label simple – “Unsubscribe” or “Opt-out” – that’s because mailbox providers scan email body to confirm the presence of that link when they are getting a large number of sends for a bulk campaign. Sometimes, just missing the unsubscribe link (like on a test email) can make a difference for inboxing.

It may be helpful to offer subscribers a way to opt out from certain communications instead of all brand communications at once. Providing this option has become even more necessary with the introduction of General Data Protection Regulation (GDPR) laws in May 2018, which states that the consent to receive emails should be explicit and not be bundled. If subscribers only opted in to receiving product updates, you can’t additionally send them promotional emails or messages on behalf of your partners.

Not all ESPs offer an easy way to setup an unsubscribe page with preferences, but it’s worth spending the time to investigate and implement it.

3. Configure Your Sender Correctly

While this step is optional with most email service providers (ESPs), probably because it’s somewhat technical, serious marketers know it has to be done, and done correctly. 

Verify your sender domain, add SPF and DKIM records

Most email platforms allow you to configure Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) for your domain to help improve your sender reputation by making your emails appear more authentic and safe. We highly recommend getting both SPF and DKIM configured for every sender domain you use to send emails. 

Use a different sender domain and IP addresses for any cold email campaigns, which have the lowest engagement and can burn your sender reputation. You may want to consider using a different sender identity for your marketing and transactional emails as well, to ensure your transactional email delivery doesn’t get affected by your bulk campaign practices.

Use a custom tracking URL in your emails

Using a custom tracking URL in your emails can help your emails appear more authentic and help with inbox placement. Consider customizing tracking URL for your emails, using the same domain as your sender address. For example, if your sender is sales@your-domain.com you can use email.your-domain.com as your tracking URL.

You can see your tracking domain by hovering over a link in your sent email message, while in your inbox. 

Example: Mailchimp’s default tracking link

Setting up a custom tracking domain will require updating your domain’s DNS records (to create the desired email subdomain and map to email provider URL) and then adding the updated records to your email provider’s platform as a configuration or setting. Many email marketing platforms allow this, although some call it white labeling and most charge extra for it or make it available on higher tier plans only. 

Examples of commonly used tracking sub-domains

4. Optimize Sender From Label and Address

Specify a useful sender (from) address

Be it a question or general feedback, email recipients commonly want to respond to the messages they receive. For this reason, it’s best if your reply-to email address is one that goes straight to your support team (or at least forwards to them). You don’t want to discourage your subscribers from responding to your message by using a no-reply address, such as noreply@your-domain.com, because replies are part of the engagement metrics that some email service providers (Gmail, for example) use for determining the importance of your email and whether it belongs in the inbox or spam folder.

Another consideration is that customers will sometimes request removal from a mailing list, and if they see noreply@your-domain.com they are more likely to hit the “Spam” or “Complain” buttons instead – complaints are the worst form of engagement you can have on your email campaigns.

Lastly, no-reply addresses run a greater risk of being sent automatically to the junk folder. As this GlockApps article states, “Some ISPs, network spam filters, and customers’ personal email security settings are set up to move messages with ‘no-reply’ addresses to the junk folder.“

In short, if you care about your email campaigns’ engagement rate and future deliverability, specify a meaningful sender and reply-to address.

Use a descriptive Sender label

While a clever subject line can certainly help you stand out in a crowded inbox, your subscribers will still need to recognize your brand’s communication when they scan their inbox. You don’t want them left wondering why they’re getting an email from someone they don’t know with a cryptic subject line that promises to solve pain point XYZ.

Consider these three basic options for sender labels:

1. Brand or Product Name
2. Brand or Product Name + Function (e.g., Support, Sales)
3. First Name at/from [Brand or Product Name]

With the third option, consider the name of the person as well as your product name. Is your product name a single word, one to two words or longer? Will the entire label fit in the “From” field? Whatever format you choose, make sure to be consistent across all of your sender accounts and test for appearance before you start using a new sender label.

Optimize your subject line and preview text

The keywords used in the subject line weigh heavily into the spam score mailbox providers assigns to an email. This is why short subject lines perform the best when it comes to inbox placement. Keep your subject line 3-4 words and continue it in the preview field.

While it isn’t always a visible element of your email’s design, preview text provides valuable real estate to expand your message. You should think of it as a second subject line – an additional opportunity to convince your recipients to open your email. The better your open and engagement rates are, the better chance you have of landing in the inbox. Many platforms, including BigMailer, support specifying this text. If your platform doesn’t support it, you can still have a pre-header element as part of your email template design. It can either be the first line of text in your email or a hidden text in the beginning of your email.

Inbox Preview Examples

Notice how in the example above 1 sender didn’t update the placeholder text with their own, don’t let it happen to you!

5. Avoid SPAM Keywords (Especially in The Subject Line)

The keywords used in the subject line weigh heavily into the spam score mailbox providers assigns to an email. This is why short subject lines perform the best when it comes to inbox placement. Keep your subject line 3-4 words and continue it in the preview field.

Here are some articles with SPAM keyword lists:

• 475 Keywords to Avoid
• 188 Spam Words to Avoid

Don’t Forget to Optimize Your Preview

While it isn’t always a visible element of your email’s design, preview text provides valuable real estate to expand your message. You should think of it as a second subject line – an additional opportunity to convince your recipients to open your email. The better your open and engagement rates are, the better chance you have of landing in the inbox. Many platforms, including BigMailer, support specifying this text. If your platform doesn’t support it, you can still have a pre-header element as part of your email template design. It can either be the first line of text in your email or a hidden text in the beginning of your email.

Inbox Preview Examples

Notice how in the example above 1 sender didn’t update the placeholder text with their own, don’t let it happen to you!

6. Segment and Customize Campaigns

Segmentation (and personalization) are key to inboxing for large volume senders, because it offers opportunity to:

1. Break down your overall list into smaller segments for more granular analysis
2. Optimize send time for best engagement
3. Optimize target audience for a tailored content/message
4. Customize and personalize content based on custom fields

All these strategies can help you inbox better, and personalized content is especially helpful for inboxing, or avoiding Promotions tab, with Gmail.

You can optimize your send time based on your target audience location. Ideally, you would capture country or timezone at the time of email capture. Some providers, like BigMailer, automatically capture subscriber’s location on their forms.

Segmentation rules examples in BigMailer

It’s not uncommon for marketers to have additional custom fields, like first name, for only a segment of the list. In this case conditional statements inside the template can help you adjust your content accordingly.

Example: Conditional email content

7. Monitor Sender Reputation

There are several parts to sender reputation:

1. Blacklists – referenced by EPSs, but monitored by 3rd party organizations
2. IP reputation – managed by ESP  and monitored by each mailbox provider
3. Sender history – maintained by each mailbox provider

Blacklists

A site/domain can also be blacklisted as unsafe with various monitoring services, like Google Save Browsing. You can monitor your domain/IP health by signing up with various providers, like Google Postmasters, or blacklist monitoring services. When you signup for ongoing IP/domain blacklist monitoring service you can get notified if/when any change occurs.

IP Reputation

IP (internet protocol) address is a unique address that identifies a device (computer or server) on the Internet or a local network. When you start using a hosted transactional or bulk email marketing service, by default, your emails are being sent from a group of servers with different IP addresses, and that group of servers is shared by multiple senders like you. So your emails use a “shared IP pool” and the reputation of those shared IP addresses is shared as well.

The biggest factor in IP reputation is whether it’s blacklisted, which happens when emails sent from a given IP get flagged as “Spam” by the recipients. If the % of complaints is high enough, the IP gets blacklisted and may get blocked by ISPs. You can benefit from a dedicated IP address you are looking for a fresh start and looking to invest in building a strong email sender reputation.

Unfortunately, low volume senders are limited in their options because most low cost bulk email marketing service providers don’t offer an option to get a dedicated IP address on lower pricing tiers. And if you use an ESP that does offer access to a dedicated IP, it may come with a hefty price tag (for example $250 per IP at ConvertKit).

Sender History

There are several factors that mailbox providers take into consideration:

1. Domain age – recently registered domains will be treated as more suspicious
2. Domain sending history – any sudden jumps (more than 20% per day) in volume point to inorganic list growth.
3. Domain historical engagement – the lower the engagement rates the lower likelihood of future inboxing

To maintain healthy sender reputation marketers have to warmup new domains with low volume sending and maintain high engagement rates by sending less frequently to inactive subscribers.

8. Test Your Email Campaigns

Example of a high-quality email and sender in spam folder

Testing every email adds an extra step, but it’s an incredibly important one. Because testing typically has to happen outside of an email marketing platform using tools provided by websites like Litmus, Email on Acid or GlockApps, you may be tempted to skip it. You shouldn’t if you have a large list or email is critical to your business – it may mean the difference between your message being successfully delivered or it being flagged as spam.

With that said, even experienced marketers using these tools can’t avoid landing an email in the spam folder every once in a while – this is especially true for Gmail and is often due to the topic of the message. Just check out an example of an email from Starbucks – probably the only version that ever landed into Spam folder in many years.

Don’t stress – if the occasional email gets a low engagement rate because a few spam keywords made it into your message, know that you are not alone. It’s becoming necessary for email marketing platforms to embed email testing into their platforms to simplify a marketer’s campaign workflow. This is exactly what the team at BigMailer is currently working on, and we hope to delight our customers with this workflow improvement soon.

Final thoughts

Implementing all of these best practices may seem like a lot of work, but many of them can be taken care of just once and offer the biggest impact. Some of them, like testing of the content, have to become a part of your typical campaign management workflow.

Following all best practices can make a huge difference in your inbox placement and subscriber engagement overtime and deliver exceptional ROI for your time and marketing budget.

Think you have this list covered and want more insight? Check out this advanced deliverability and inboxing article.