Email Deliverability – A Guide to Best Practices

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.

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A hard bounce typically means an email address is no longer valid, but sadly it can be a result of the mailbox provider service being down and not responding with a correct response code that would be correctly classified as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.

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 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 % of soft bounces – when the rate is extremely high (over 60% per provider or over 20% overall).

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.

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

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. 

1. Maintain list quality

Email lists decay at an average rate of 2-3% per month – 2% for B2C and 3% for B2B lists. The rate is higher for B2B lists because people change work emails more frequently than personal emails, due to job changes. 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.

In BigMailer, a bulk/marketing email campaign with a bounce rate over 8% (default, configurable by customer) is automatically paused to protect sender reputation.

Old and unengaged lists should be verified before being imported into an email marketing platform if not engaged in more than 2 months.

We have partnered with low-cost email validation services Bouncer (recommended for customers in Europe) and Emailable to get our customers a discount on their services. See our email verification provider comparison article for more detailed info.

Verification Process and Results

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.

2. Include a plain-text version of your email

While only a tiny fraction of your audience may see the value in a plain-text email, SpamAssasin, a widely used spam tool, considers it essential. The tool assigns two points to your email if no plain-text version is available. If your message reaches a score of seven, it’s identified as spam, so two points is a pretty big penalty for something that’s fairly easy for a savvy marketer to execute.

While some email platforms will help auto-generate a plain-text version from the HTML version, most of them don’t require one. If you have to create a plain-text version yourself, don’t cut corners – it should match the message in the HTML version and not be much shorter. Check out this article from Litmus for more guidance on formatting plain-text emails.

3. 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. 

Example: unsubscribe page with preferences format

Example: unsubscribe page with preferences format

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.

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. e.g. If subscribers only opted in to receiving product updates, you can’t additionally send them promotional emails or messages on behalf of your partners.

4. 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 preheader 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

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. Manage sender reputation by keeping low complaint rate

Just like with an IP reputation, the sender reputation is affected by email recipients marking the emails as “Spam”. An industry benchmark is to keep complaint rate below 0.1%, but it’s more nuanced than that. The lower the volume of sending the less sensitive a mailbox provider is to the complaint rate, due to statistical insignificance (just 1 complaint can change the ratio dramatically). High volume senders have to be more vigilant – being close to the 0.1% ratio can trigger a manual campaign review from an ESP they use and can be followed by an account suspension.

When an Unsubscribe/Opt-out link is missing or is small and hard to find, the complaint rate tends be higher.

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.

6. Configure DKIM and SPF for you sender domain

While this step is optional with most email service providers and requires a little more technical knowledge than the other items in this guide, 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.

7. 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, 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 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.

8. 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.

9. Test emails with spam tools

Starbucks email in spam folder

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.

10. 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 you can use as your tracking URL.

custom tracking URL

Examples of commonly used tracking sub-domains

This 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. Most leading 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. 

11. Monitor your engagement

Mailbox providers keep track of how your email recipients interact with your emails, so you need to maintain a high engagement to achieve a high inbox placement rate. Consider these tactics:

  1. Periodically remove unengaged subscribers from your list. Identify anyone who hasn’t engaged with (didn’t open) your emails for 90 or 120 days and either permanently remove them, or exclude them from your frequent mailings (you could send them your highest engagement/appeal content or only critical service notifications regarding their accounts).
  2. Send emails with appealing call-to-actions (CTAs) and make it necessary to click and interact with your email. It may seem obvious, but it’s not uncommon for some marketers to include phone numbers to place orders or have the website address in giant font to make it memorable, which may push a user to simply type the address when they are ready to place an order instead of clicking on a link in an email (and for a marketer to correctly attribute a conversion or sale to the email sent).
  3. Engage in email exchanges with your customers using the same email address that you use to send emails, or at least using the same sender domain, for example for customer service inquiries. Any email interactions between your and your customers signal trust to mailbox providers.

12. Consider using a dedicated IP address

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 IF:

  • You are looking for a fresh start and looking to invest in building a strong email sender reputation
  • You have a small list now, but plan to grow it fast and monetize it, while controlling cost and ROI by using a low cost provider
  • You have built up a sizable list (over 20,000 subscribers) and ready to engage it with high quality content

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).

Final thoughts

Implementing all of these best practices may seem like a lot of work, but many of them (e.g. sender configuration, email verification) can be taken care of just once and offer the biggest impact and ROI improvement on time spent. 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.

We hope you found this guide helpful or learned something new. Happy email marketing!