Email Delivery Guide Part 5: Writing Good Emails And Avoiding Spam Filters

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, 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. Part 2 discussed how IP addresses, sending volume, DNS and authentication affect deliverability. In Part 3, we looked at how to maintain email lists, bounce handling and spam complaints. And Part 4 covered how to handle unsubscribes when they happen, recipient engagement and whitelisting. In this fifth and final installment, we’ll take a look at how to write good emails that don’t trigger spam filters.

There are a few tricks to remember about content besides the mantra “sending something people want.” One of the things Mailgun offers our customers is the ability to set up a test mailbox at Mailgun and enable our spam filters to receive a “spamicity” score to test how your content is judged by spam filters. Absent that, here are a few tips to consider.

  • Personalize your emails to each recipient. Ideally, the content should reflect recipients’ specific interests or usage patterns in your application. At least address them by their name…don’t be rude! Mailgun has recipient variables that you can define and use with your email templates to achieve detailed levels of personalization.
  • It is best to send multi-part emails using both text and HTML, or text only. Sending HTML only email is not well received by email service providers (ESPs). Also, remember that ESPs generally block images by default so HTML only will not look very good unless users are proactive about enabling images. There are a few tools available to test how your email will render across ESPs and browsers. Litmus offers one, as does Return Path.
  • The higher the text to link and text to image ratios, the better. Too many links and images trigger spam flags at ESPs.
  • Misspellings, spammy words (buy now!, Free!) are big spam flags, as are ALL CAPS AND EXCLAMATION MARKS!!!!!!!!!!!!!
  • The domains in the from field, return-path and message-id should match the domain you are sending from.
  • Make sure you are using unsubscribe links and headers in your emails. Many ESPs (particularly Hotmail) pay attention to this and if they are not there, you are likely to get filtered. You can always use Mailgun’s auto unsubscribe handling if you don’t want to deal with this on your end.
  • Gmail pays particularly close attention to Message ID and Received headers. Message IDs that are formed incorrectly (without brackets <> and with wrong domain after @) can make Gmail think you are a spammer. The simplest way to create the right Message ID with Mailgun is to not include one. Then Mailgun will create a perfect Message ID for you.
  • Links should include the domain that is sending the email. Also, popular URL shorteners can be a bad idea because they are frequently used by spammers
  • A/B test your emails to optimize recipient engagement. Subject lines are particularly important. You can use Mailgun’s tagging and tracking statistics to measure A/B testing and improve your content.

So that’s it. We hope that this Email Best Practices Deliverability guide will help you send better emails and make sure they are delivered — no matter which provider you use. Happy Sending!


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