It seems likely your site had something detected as malware, or is still being detected as malware. When I test sending a mail to my M365 account with your URL

It seems likely your site had something detected as malware, or is still being detected as malware. When I test sending a mail to my M365 account with your URL mentioned I find that it gets quarantined (same as if I try to send an email from my M365 account with that URL). In your M365 test tenant, you should be able to go https://security.microsoft.com/quarantine https://security.microsoft.com/quarantine and see that the emails are getting quarantined, with this information provided as to why: Detection technologies: URL detonation reputation, Mixed analysis detection Given that it says "URL detonation reputation" rather than just "URL detonation", that suggests it's using historical information rather than having performed a new test. This is Microsoft Safe Links functionality - at the very least since you should be able to find the quarantined emails, the headers will contain a correlation ID support can use, although they might not have much power over safe links.