After a couple of users complained, I discovered that since Monday our Discourse instance emails (via Mailgun) no longer seem to be getting through to those on Microsoft email tenants. This includes enterprise and general (e.g. hotmail.com, outlook.com).
On testing our deliverability, we didn’t have DMARC set up. This has been fixed, and we have a near perfect deliverability score now on mail-tester.com:
Your message does not contain a List-Unsubscribe header
We don’t have any customisations for our email template.
Traffic to Gmail and other services seems fine; it is just Microsoft, and this seems to be continuing 24hrs after the DMARC fix.
Of note, Mailgun has put out this notice about Microsoft and email:
In summary, if DMARC is set up, we should be fine (but currently are not). While this was meant to kick in on 2025-05-04T12:00:00Z, there is an ominous note that:
Later (date TBD): Expect full rejections for senders who remain non-compliant.
Has anyone else noticed an impact of this with their outgoing emails bouncing? Any suggestions as to how to proceed from here?
If I understand correctly, the lack of List-Unsubscribe header is just a suggestion from mail-tester.com, not necessarily the reason for the rejection by Microsoft. My guess is that the rejection is more likely to be related to your having sent emails without DMARC configuration in recent past.
When you check your Mailgun logs, what responses do you see from Microsoft for the emails that failed to be delivered?
I didn’t even know that Mailgun stores logs too - I guess that makes sense! First time I’ve had to delve in there.
This is the rejection reason for almost all of them:
4.7.500 Server busy. Please try again later from [.27]. (S77719) [SY2PEPF00004FF3.ausprd01.prod.outlook.com 2025-06-27T03:22:50.770Z 08DDB10C5071CA89]
And one a slightly more worrying from hotmail.com:
4.7.650 The mail server [.27] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S775) [Name=Protocol Filter Agent][AGT=PFA][MxId=11BB6463522E609D] [CY4PEPF0000EDD1.namprd03.prod.outlook.com 2025-06-27T03:22:41.176Z 08DDB231AE10F67F]
As we seem to have a shared IP address for sending email via Mailgun, and this seems to be the culprit, I’ve put a support ticket in Mailgun. Will report back.
