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.
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.
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.
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.