[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.