Such a tricky question! Getting your messages delivered to the spam folder can be super frustrating, but with these tips, you should start seeing better delivery results in no time!
One of the biggest things to be aware of is the recipient's server's settings and spam filters - this could be the main reason it's occurring and there's limited options to correct it from the Mailgun side.
Have Recipients Safelist Your Domain
If the reason why your messages are being delivered to a recipient's spam folder is due to the server's settings, the quickest way to bypass these would be to invite your recipients to safelist your sending domain (found in your Mailgun Control Panel) and/or the sending IP. Please note though, if you're not utilizing a dedicated IP for your sending, there is a chance your sending IP could be changed automatically by our system. Safelisting the sending IP should only be a secondary option to safelisting the sending domain!
Check Message Content
So about those spam filters - they take a large number of things into account that we aren't able to see due to their own security trying to protect their environment and users, but message content is something can be be edited to improve your inboxing! The best way to determine what might be flagged in your messages would be to review them through any 3rd party application to test the "spamminess" of your content and what can be improved upon.
Check DMARC Compliance
Additionally, another thing to be aware of is DMARC. This policy helps any ESPs (Email Service Providers) you're sending to know that your messages have been protected by SPF and/or DKIM records - in this case, both! However, it also indicates what should happen to a message if it does not pass the DMARC check - either mark it as spam or reject the message completely! To ensure your messages pass the DMARC check with flying colors, you'll need to make certain the From: address reflects the same domain as the domain that is sending the email.
For example, if your sending domain is ABCdomain.com, but your From field is set to 123domain.com instead, you will likely run into these errors in the future. Email best practices are clear that the domain of the From: address and sending domain should match!
And finally, we do have a plethora of articles that are devoted to helping you improve those delivery rates! Be sure to check them out! Here's a sampling that should get you started: