Reliable email delivery is critical for most companies that do business over the Internet. While users can opt to be contacted through other mediums, email is still the primary mechanism for providing transaction confirmations and the basis for account registrations. There are two categories of email. Transactional emails are those which a business sends to a single user in response to an action that the user performed, like a purchase, registration or friend request confirmation. In these cases, the user expects to receive the email. Transactional emails are contrasted with marketing emails, which are generally sent in bulk to a large number of users in order to promote something. Most marketing emails include a call to action that drives incremental business. If users aren’t receiving transactional emails, they will be missing important information or will be blocked from further action. If they aren’t receiving marketing emails, then they can’t generate incremental business for the sender.

Email deliverability is represented by the percentage of a sender’s email which makes it into the user’s inbox. Most consumers maintain a personal email account with one of the major email ISP’s – Google (Gmail), Microsoft (Hotmail, Outlook), Yahoo (Yahoo Mail), Comcast (, etc.  These ISPs have developed sophisticated systems to examine all incoming email and determine which should be allowed into their users’ inboxes. With the widespread prevalence of email spam, this has become a necessity.  A very small percentage of all email is actually delivered into the average user’s inbox. Email not delivered to inbox is either discarded entirely (blocked) or directed to the Spam folder (also known as Junk or Bulk). Additionally, ISP’s will throttle the amount of email they allow into their network from a particular sender. If a sender is transmitting a large volume of email to an ISP and experiences throttling, then there will be a significant delay between the time their systems initiate the email send and the user actually receives it. As an example, this could impact new user registrations as users await their account validation email.

If your business depends on reliably delivering email to your users, then it is very important that you understand how to manage your email program. You can outsource this function to  a number of email service providers, like SendGrid, MailChimp or Amazon’s Simple Email Service.  These vendors will handle most of the work related to maximizing your email deliverability.  Even if you outsource this function, you should understand how email deliverability works in order to track the success of your email program.

This post will provide some background on the email distribution process and tips for maximizing deliverability of your company’s emails. While your peers in marketing will track the impact of email campaigns closely, the engineering team still needs to manage the mechanics of the email infrastructure.


In order for an email to be sent, it is transmitted by a mail server. Each mail server will have one or more associated IP addresses. The sending IP address is an important aspect of deliverability. For a particular sender, these IPs will be fixed and change infrequently.  This is because the sender IP address builds up a “reputation” over time.  Reputation can be thought of like a trust score, representing the aggregate of past email behavior for that IP. One email marketing vendor, ReturnPath, has established a Sender Score that is used by many ISPs to check the reputation of an IP address.  Sender scores represent a value between 0 and 100, where a higher value is better.  A Sender Score over 90 is considered good. Each ISP receiving email will build a profile of the source IP over time. They may check Sender Score initially and then supplement that with their own proprietary algorithms for reputation over time. The higher an IP’s reputation, the more likely email from it will be accepted at high volume by an ISP and delivered into users’ inboxes.

When a new IP address begins sending email to an ISP’s users, the ISP will have no history on this IP.  As a result, it will throttle the amount of email allowed from this IP.  This throttling will occur across all email sent from the source IP, regardless of the relationship between individual users and the company. Therefore, if you are just beginning to send emails to your users from a particular IP address, the allowed volume will be low. Alternately, if you have been sending a large amount of email from a particular IP historically and want to shift this traffic to another IP, you will need to “warm up” that new IP. Warming up an IP involves gradually increasing the volume of email sent from it, until you reach the normal send rate.

Most companies sending a large amount of email will maintain multiple source IPs. The number of IPs from which you send email can vary based on a few considerations. First, you should separate the IPs from which you send transactional versus marketing email. This is because you want your transactional email IPs to have the highest reputation and thereby the highest chance of landing in the inbox.  Since transactional email is expected by your users, they are less likely to mark these emails as spam.  Once you divide your sending IPs into two pools for transactional and marketing emails, then you will want more than one IP in each pool.  This is because a single IP can sometimes have a temporary issue with an ISP and be throttled.  I have seen ISPs throttle only one IP out of a pool of 5 IPs randomly, even when all IPs have similar reputation levels and send rates. When a single IP is throttled, your mail servers should automatically shift email sends to other IPs. Given this need for IP redundancy, each pool should have at least 2-5 IPs. Deciding to have more IPs than 5 will be influenced by your overall email volume.  You need enough email volume passing through the IPs to register with each ISP, including the smaller ones. I’d recommend at least 5MM emails a day on each IP.  If you aren’t near these volumes, then limit your sending IP pool to the minimum (2-3 IPs).

On the other end of the email send is the individual user. Every user has a relationship with each of the companies that send them email. ISPs will also track this relationship – whether they have observed email from your company (source IP) being delivered to each user in the past. As users form new relationships with companies, there is the risk that the ISP categorizes the new email as spam. This can be mitigated if the source IPs have a high reputation.  Beyond the reputation of the sender’s IPs, the ISP will track the engagement rate of each user with each sender’s emails.  They will track whether the user is opening the emails and clicking on any links inside.  This data is fed back into their algorithm for determining if a sender’s emails should be delivered to users’ inboxes consistently.

In addition to these influencers, there are a few other actions which will affect email deliverability:

  • Problems with the user’s email account.  The state of an individual’s email account can change over time. They can close the account, or their ISP closes it after a long period of inactivity. Alternately, the email account storage allocation can fill up, where there is no longer space available to receive new email. The ISP might even be experiencing a service issue with its email infrastructure. When there is an issue with an individual email account that prevents it from receiving new email, the ISP will generate a “bounce” and send that notification back to the sender. This bounce can be captured by your mail servers and processed. Bounces will have codes identifying the type.  As the sender, you can examine these codes and determine how to treat the email account in your system for future sends.
  • User unsubscribes from your email.  The unsubscribe action is usually initiated by the user as part of following a link on your email or site, where they are requesting to be unsubscribed from your email sends.  These unsubscribe requests should be processed as soon as possible.
  • User marks your email as spam. Users can mark your email as spam.  This indicates to the ISP that they no longer wish to receive it and provides an indication to the ISP that the user considers the email to be spam. Users have a wide range of definitions for spam, which makes handling of spam reports tricky.  Generally, an ISP will examine all the spam reports for a particular sender IP in aggregate and compare that to the amount of email delivered to inbox. Maintaining a low spam to received email ratio is very important.

These actions can all be tracked as part of the feedback loop that you set up with each ISP. Through the feedback loop, the ISP will send notifications back to an address you specify.  Your mail servers will receive these feedback messages. You can write scripts to process them and make appropriate changes to the status of each user’s email account.

Maintaining your Email Reputation

After the relationship between your source IPs, the end users and their ISPs has been established, there are actions you can take to maintain or improve your deliverability. This guidance boils down to two areas. First, keep your email list clean.  Second, make sure that users want the emails you send.

  • Use double opt-in for gathering email addresses. Sending email to addresses which actually exist is an important contributor to maintaining the reputation of a sending IP. Conversely, if an ISP sees an IP sending email to addresses that don’t exist or are inactive, it is strong indicator of spam activity. Confirming the validity of an email address can be accomplished through the double opt-in approach. Double opt-in is performed at the point of user registration, and involves requiring the user to click on a link in an email sent to the user’s email account. This confirms that the user has control of the email address.  Confirmation of email serves two purposes.  First, it indicates that the email address exists and wasn’t entered as gibberish by an unscrupulous user trying to get through the registration process quickly.  Second, it prevents someone from registering with a different person’s email address. In both cases, future emails sent to that address would impact IP reputation. Double opt-in eliminates these risks.
  • Process bounces. When the ISP generates a bounce for an email address, ensure it is processed. You can write scripts that examine each bounce and determine how to handle them. There are two types of bounces – soft and hard. Soft bounces indicate a temporary issue with the account, usually a problem with the ISP’s infrastructure or a full email account. Hard bounces represent an email address that has been deactivated or doesn’t exist.  Hard bounces should disable future sends immediately. Soft bounces can allow for a few repetitions before being disabled. As a follow-up, you could present an alert to the user the next time they log into your application, indicating an issue with their email account. Continuing to send email to a problematic address that is generating bounces will impact your IPs’ reputation.
  • Do not trust email addresses from another source. Email addresses for your core customer list must represent actual customers who have opted into your service. Do not import email addresses from another source into your primary email list. If you are acquiring another company and need to adopt their email list, then I would send test emails from another set of IP addresses, which request the users to confirm that they still want to be contacted for your company’s offerings.
  • Process unsubscribe requests. Make the unsubscribe request easy to locate and process these immediately. While most ISPs allow a few days for unsubscribe requests to be processed, you should stop sending emails to unsubscribe recipients as soon as possible. If a user goes to the trouble of requesting an unsubscribe and you continue to send them email, their next step is likely to mark your emails as spam.
  • Authentication. Standards are available that allow an ISP to verify the authenticity of the sender when an email is received.  These include DKIM and SPF.  Many ISPs are now using authentication as an additional filter criteria. Make use of these standards in your email transmissions.
  • Provide easy update to an email address.  It may go without saying, but you should provide an account maintenance section within your application, where users can update the details of their account. Within this, make it easy to find the section for updating one’s email address.
  • Allow users to specify what types of emails they want. If your company sends different types of emails, then you should allow your users to opt out of some of them. This applies particularly to the distinction between transactional and marketing emails.  They need to receive transactional emails, but may not want to receive your promotional emails. On a section of your application’s user account settings, list the types of emails a user can expect.  Allow the user to deactivate receiving certain emails. If you want to be more conservative, you can begin with all marketing emails deactivated and ask users to opt into the ones they want to receive.  A good time to set expectations around email types and frequency is within an initial welcome email following registration. The key to these approaches is make the user feel that they are in control, which will lower the chances that the user would mark your email as spam.
  • Taper sending email to inactive users. Your application likely collects data on the last time a user logged in. You should track this usage and apply it to your email send frequency. If a user becomes inactive, then you should reduce the frequency of promotional email sends to them. For example, if you normally send an email daily with some sort of news summary or a set of recommended purchases, then you could reduce the frequency to every other day or less, as you see their usage reduce. This reduction of frequency lowers the chance that an inactive user will mark your emails as spam, rather than unsubscribing.
  • Increase engagement levels. ISPs will measure the engagement level of your users with the emails you send. That engagement level will be applied to your reputation as a sender. In order to address this, you should try to maximize the likelihood that a recipient will open and interact with your emails. Therefore, you should carefully consider the content of each email you send to your users.  Is the content relevant for the recipient? Is it fresh on each send, or are you sending the same content repetitively, assuming the user hasn’t considered your offer in past emails?

Obviously, some of these recommendations can cut into your business. For example, by sending fewer promotional emails, you will get less response. You should monitor the health of your email program closely. Try to maintain a balance between engaging your users through email and the reputation associated with your sender IPs.  If you are managing your email program yourself, there are vendors who will help you monitor your email sender reputation.  Mentioned earlier, ReturnPath collects data on the reputation of your mail server IPs from the ISPs directly.  This allows them to produce reports showing your IPs’ reputation by ISP.  Additionally, they can monitor the delivery of your email to a set of test accounts they maintain at all ISPs.  You can be alerted if delivery of a particular email type drops unexpectedly.  They also provide feedback on email design, as a means of improving user engagement with your email messaging.

Other Danger Areas

  • Spamtraps.  A spamtrap is an email address that doesn’t represent a real user and is set up solely to identify spammers. Spamtraps are usually maintained by ISPs or third party spam-fighting organizations. The spamtrap email address is generated in one two ways. First, it can be created by the ISP and then sprinkled across the web in locations where spammers would normally harvest email addresses through scraping (forums, social sites). Second, the address can represent a real account that has gone inactive. In this case, the ISP will close the account and then return bounces for several months to senders. Finally, they will stop sending bounces and record any new sends to the email address. It is at this point that the spamtrap is active. Emailing to a spamtrap is considered to a grave offense.  Even a single send out of millions of good emails can trigger blocking of the sending IP or cause a significant impact to reputation with that ISP. Spamtrap addresses will get onto your email list by either adding unverified email addresses or not processing bounces.
  • Back-up infrastructure. Most large Internet-based businesses have redundant infrastructure to handle failures. If you are maintaining this kind of redundancy, ensure that email send volume is balanced across all mail server infrastructure.  Practically, this means you want to avoid a situation where a failure in primary infrastructure results in a dramatic increase to email volume for the back-up (like from 0). This surge will likely be met with throttling by the ISPs, which will take several days to work through. It’s best to try to route equal amounts of email through all IPs that might be active at some point.