Strategies to try to avoid being detected as SPAM / Marketing

Code Changes

Implement rate limiting for sending large amounts of email at once (ie. Event Invitations)
Create subdomain for sending other than mg. and use sending email address that matches (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).
Confirm that the domains in the from field, return-path and message-id match the domain you are sending from.
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. Verify that we are having Mailgun generate the Message ID in all scenarios.
Ideally, all links should be from the domain that is sending the email.

Email Template Changes:

Remove Repmail comments from html templates.
Try switching to another responsive HTML email template or building a custom one.
Make sure unsubscribe links and headers are present in all emails. Many ESPs (particularly Hotmail) pay attention to this and if they are not there, you are likely to get filtered.
Try to lower image to text ratios.
Host images from same domain as emails are sent from.
Supply plain-text alternative version of message if not doing so already.
Check spamicity score (set up a test mailbox at Mailgun and enable our spam filters to receive a “spamicity” score to test how your content is being judged by spam filters.)

Info for ESPs:

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