[amanzi]

Have you looked at the Exchange Online and Defender portals for clues? Especially in the Message Trace section of Exchange Online?

I recently helped troubleshoot a similar issue - we were suddenly getting emails disappearing when sending to M365 customers. No spam or quarantine, just disappearing down a black hole like you described. We sent a test message to a M365 customer who could help run the message trace, and we discovered that the SVG logo in our email signature was being flagged as a phishing attack. We had been using this logo for about a year without any issues, but suddenly Microsoft just decided to block it without warning.

[joaopbnogueira]

Yes, I have.

Message Trace is an interesting one.

If we send an email without the 25friday.com keywords everything works fine, the message is shown on Message trace as delivered and the recipient gets the email with SCL 1 (all good here).

As soon as the very same email is appended with a www.25friday.com, Message Trace still shows the outbound email, also as delivered, but the recipient (if a Microsoft account) never gets the email. We used one of these emails (EML) to create a submission on the defender portal of a false positive, but they always simply disappear with 0 feedback (and the problem still occurring).

We also had a signature with the www.25friday.com link on it and took it out after realising it was causing emails to go to this black hole, so that at least we can still send emails, but we keep having to be careful to never sending any content (or attachment) that somehow mentions the 25friday.com domain.

[4ggr0]

Are you sure that it's MS who's blocking? Just think it's weird that they show it as delivered to recipient, sounds like they send the mail and something weird happens after.

[jeroenhd]

With excessive spam scores, Microsoft silently ignores your email. Just swallows it up, reports it as delivered, but never actually does, not even in your spam folder.

It's possible that this is a technical issue or a submission server issue, but it's not uncommon for Outlook to make email disappear.

[joaopbnogueira]

This sounds like what's happening to us, but only if the dreaded 25friday.com keyword is included on the email content

[joaopbnogueira]

Well, it works fine for every single recipient unless it's a Microsoft one.

If the recipient is using a personal microsoft / outlook / hotmail account the email gets delivered with a spam score of 9.

If the recipient is a MS 365 account, blackhole it is.

We even set up our own MS 365 to prove this.

[amanzi]

Sorry, I meant have you looked at the message trace on the recipient's end?