Mailgun: Messages "Accepted" but taking long time to be delivered (or not being delivered)

If this is happening regularly, it is very likely Mailgun has you on one of their low-tier IP addresses. I imagine this is the default for free accounts, since they don't want to "pollute" their good addresses with new users who may not be serious / legit.

You can check the "quality" of the IP address at a site like Sender Score. Find this line in the Mailgun log of a delivered message:

"sending-ip": "XXX.XX.XXX.XXX"

If Sender Score shows a score in the 70s, that's your problem. Send Mailgun a support ticket, as Chris suggested, and see if they can get you onto a higher quality IP address. I did so and my emails are now being sent from an IP address with a score in the 90s. Haven't had a single delay since.

Whenever a message shows as "Accepted" in the Mailgun logs this indicates that Mailgun has accepted the message and queued it for delivery. The message should be delivered fairly quickly, however it can be queued for a bit of time if you submitted a large amount of messages at once or if the recipient ESP is throttling messages from the IP/domain on your account.

I'd recommend opening a support ticket via your Mailgun account and provide some of the message-ID's so the support team can investigate the exact cause of these delays once the message is submitted to Mailgun.