Email Deliverability Guide Part 1: Overview, Reputation And Hosting

This Email Deliverability Best Practices Guide was originally published by the team at Mailgun. Mailgun is an easy-to-use, API-based email deliverability tool for developers which Rackspace acquired in August 2012.

Sending email is easy. Having it delivered consistently is hard. This blog series is a summary of email best practices that we have learned from managing mail servers for thousands of customers and from sending a lot of email.  The objective is to help outline what you need to do to have your emails delivered whether or not you use Mailgun. Of course, if after reading the guide you decide that you have better things to do than maintain email servers and manage email deliverability, we’d love to help!

In this guide, we will not focus on the content of email messages. It’s as simple as this: send something people want (to paraphrase Y Combinator’s motto). That’s probably the hardest part, so apologies for that huge caveat. Instead, we’ll focus on the infrastructure and monitoring of email so that if you are sending something people want, they will get it, and if you are not, you will know about it and hopefully change that.

In Part 1 of this guide, we look at how to maintain a good reputation with email service providers and ensure that your email infrastructure is hosted properly for maximum deliverability.


One of the most important assets you have in the email world (much like the real world) is your reputation. If you do not have a good reputation tied to your domain and your IP address (IP used herein for abbreviation), your email will not reach your recipients’ inboxes. Due to its popularity and its unique ability to push information to users, email has been overrun with spammers (as if you didn’t notice). Depending on your definition, approximately 90 percent of all email is spam, according to MAAWG. Due to this, email service providers (ESPs) like Gmail, AOL, Yahoo and MSN/Hotmail have declared an all-out war on spammers. This has made our inboxes more pleasant places. This also makes it very important to manage your email reputation. If it is not impeccable, you will get caught in the ESPs’ spam filters.

A good analogy for your email reputation is your personal credit score. Obviously, a bad reputation will hurt you. However, not having a reputation will also hurt you. If ESPs don’t know you (or more specifically your IP and domain) they will assume the worst and filter you, at least initially. It’s tough to blame them given all the spam out there. Due to the importance of reputation, a significant portion of our discussion on best practices revolves around building and maintaining your email reputation.

Beyond making sure that the infrastructure is properly set up (more on this below), it’s important to listen what ESPs are telling you if they are not delivering your messages (Mailgun provides this information to our customers. Check with your email provider or review your logs if you are not a Mailgun customer)

  • Are emails being delivered and if not, why?
  • Is a recipient ESP throttling your traffic and why?
  • Are messages bouncing due to incorrect domains or stale addresses?
  • Are recipients unsubscribing or complaining of spam?
  • Are recipients engaging with your emails by opening them and/or clicking on links?

You should use all of this data to make sure that you are complying with ESPs’ guidelines and adjust your email sending to stay in their good graces.  At Mailgun, we use queuing algorithms that adjust sending rates based on feedback from recipient ESPs. You should consider this as well.

Some email service providers use FUD (fear, uncertainty and doubt) about email deliverability to sell you a deliverability fairy that magically gets your emails to the inbox. This is most definitely not the case and your actions, as the email sender, play the biggest part in good deliverability. If you’re a Mailgun customer, we give you all the tools for establishing a good sending reputation, but it’s ultimately up to you to send emails appropriately.

Regardless of how you send your emails, if you follow a couple rules (along with properly authenticating your email), you will most likely build up a great email sending reputation:

  • Only send emails to people who have signed-up to receive them from your website/application/service, and always first send a confirmation link to confirm their address is correct (aka “double opt-in”).
  • Track your email and adjust your sending based on feedback from ESPs and recipients (e.g. don’t send additional emails to recipients that have unsubscribed or complained of spam).


A brief note on hosting: since hosting technology is changing so quickly, it will likely be out of date in a few months. Due to the way that ESPs currently assess the reputation of email senders based on IP address; large, virtual cloud environments are generally not the best environments for email. This is not due to the reliability of the cloud itself, but the methods that ESPs use to fight spam. To overcome these limits:

  • The IP address should be static so that your domain(s) & IP address(es) build a reputation together. Also, some more strict recipients ESPs may require whitelisting your IP address. Unfortunately, these should be IPv4.
  • The IP address and surrounding IP addresses should have a good reputation. This is often a problem at large cloud environments due to their ease of use and lax monitoring (which is inviting to spammers).
  • Mail Transfer Agents should ideally be on  dedicated (non-virtual) machines, optimized for I/O.

How do we host our email infrastructure to get the best results? We host Mailgun mostly on dedicated servers. We do use cloud servers for some of the infrastructure (where it makes sense), but for most of Mailgun we like large, robust, dedicated machines for the reasons outlined above.

We use dedicated IP addresses in large subnets and we do background checks and extensive testing on our IP addresses. Because they are in large continuous blocks, they are less likely to be affected by other, external IP addresses. ESPs and blacklists occasionally block entire subnets if any of the IPs have questionable reputations. So even if your IP is clean, it might be blocked because of surrounding IPs. Larger subnets mitigate this risk.

We dream of a day when IP reputation does not matter and we can rely on domain reputation, but unfortunately we are not there yet.

That’s it for today.  Stay tuned for more email deliverability tips.  Next time, we’ll discuss managing IP address, DNS and Authentication to maximize deliverability!



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