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 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.
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 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.