Email Deliverability Guide Part 2: IP Addresses, DNS And Authentication

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. 

In Part 1 of this series, we gave an overview of how to maintain a good reputation with email service providers (ESPs) and how to host your email infrastructure if you want to manage email deliverability yourself. Today, we’ll look at the important topics like how IP addresses, sending volume, DNS and authentication affect deliverability.

IP Addresses and Sending Volume

If you send a lot of email (greater than 50,000 per week), it is a good idea to have a dedicated IP in order to isolate your reputation. If you share your IP, you share your reputation with those other senders. In addition, ESPs rate limit your emails based on the IP. So if you are a high volume sender you should consider getting a pool of IPs. However, your reputation can also be hurt if you are not sending enough volume consistently from an IP, so it’s a tricky balance.

If your email sending is volatile with large spikes of volume, ESPs may assume those large spikes are spam. Also, if your overall volume is too low, they won’t acknowledge your reputation. Generally, if you send less than 5,000 emails per day, a shared IP may be the right solution.

The other thing to consider is using separate IPs for your bulk and transactional mail if you are sending high volumes of email. There are a couple reasons for this:

  • Delivery of time-sensitive transactional emails may get queued behind a large batch of bulk/marketing emails.
  • Your transactional mail will be affected by the reputation created by your bulk/marketing mail.

Even if you have a clean IP address, you need to warm up the IP gradually. This means sending emails at a low rate initially and then gradually increasing that rate, taking into account ESP feedback. If you send a ton of emails right away, they will get filtered or dropped by the ESPs. In some cases, they won’t even tell you they are dropping them.

If you decide to use Mailgun, we offer both shared and dedicated IPs. We constantly monitor the traffic on these IPs. So even for shared IPs, you can be comfortable that your reputation is not unduly influenced by others. We also offer pools of IPs for high volume senders. In addition, we have queuing algorithms that gradually warm up your IPs. Our sending rates automatically increase over time as your IP warms up. Finally, we separate our sending queues for each domain you set up at Mailgun, which mitigates the need for multiple IPs for different types of traffic.


Your email reputation is not only tied to your IP, but your domain name (DNS stands for Domain Name System) as well. You should keep this in mind as you set up your email infrastructure. For the same reasons as above, it is a good idea to have separate domains or subdomains for your marketing, transactional and corporate mail. We suggest that you use your top level domain for your corporate mail and use different domains or subdomains for your marketing and transactional mail.

While it is not required to use the same domain in the From field of the message as the actual domain sending the message, it is highly recommended. Hotmail is especially finicky about this requirement and has a higher propensity to filter your messages to junk if the two domains do not match.

You should also make sure that you use a well-regarded DNS provider and that you publish all of your contact information in the WHOIS record. If you hide your contact information through a proxy, ESPs may take that as a signal that you are spamming.

Also, make sure you include the appropriate records at your DNS provider for authentication (see below). While it’s not required to point mx records to the same domain as you are sending from, it is recommended. There are email providers (albeit, a minority) that will check if mx records for the domain are valid before accepting email.

For Mailgun customers, you have the ability to create multiple domains or subdomains very easily. You are free to create multiple domains and subdomains for each of your transactional, marketing and corporate email. Each domain has an isolated queue, so your transactional emails won’t get held up by your bulk mailings.


It is very important that you use the appropriate authentication methods with your email. If you do not authenticate your email properly, ESPs will assume you are spamming and will filter or just drop your email.

The common types of authentication are:

Mailgun uses all of these types of authentication. When you sign up for Mailgun, we provide the appropriate records for you to include at your DNS registrar. We also provide a verification button you can use to make sure that your records are set up correctly.

That’s it for this time! Let us know if you have any questions in the comments. In part 3, we’ll discuss managing Email Lists, handling Email Bounces, and Feedback Loops.



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