Note: This article is not sponsored and I am in no way affiliated with Glock or Postmark.

This is a quick tip for those who suffer from deliverability issues to Microsoft domains, specifically outlook.com, live.com, and hotmail.com. The emails arrive but end up in the spam folder. If you cannot seem to figure out why and everything seems fine then perhaps the issue is with Mailgun itself. The only clue in the email headers is this: X-Forefront-Antispam-Report: EFV:NLI;

Before you do anything drastic then I suggest:

1. Use https://www.mail-tester.com/ to figure out if the content in the email is fine.
2. Double check your SPF/DKIM setup

If neither of those help then perhaps the issue is with Mailgun itself. A simple way to figure this out is to sign up for Glock and run some tests. What Glock does is that they send email to a bunch of email addresses using your SMTP and contents. These email addresses are controlled by Glock and they will report any deliverability issues. This will clearly show if an email ends up in spam for example. If that is the case and there are no obvious errors then I would suggest trying a different provider. In my experience Mailgun will not do much about this except rotate the sending IP which will probably not help.

Instead, I suggest signing up for Postmark instead to replace Mailgun and running some tests using Glock. In my experience Postmark will deliver email to Microsoft domains with no problems.

The issue here is that sending IPs are shared by a LOT of people and it doesn't take much to "dirty" that IP address. A dedicated sending IP address is not always a good solution either because you will probably run into other issues. Postmark seems to take greater care of their IP addresses somehow and that is why they don't have this particular deliverability issue.

Happy emailing!