TailScale VS smtp.office365.com

Be careful when sending email from a non-domain PC that has TailScale installed.

No domain means that it has no DNS suffix, so TailScale will “help” by adding the PRIVATE DNS suffix

Office365 will refuse to accept email that comes from a non-existent domain

The quick solution is to turn off TailScale DNS. Right click on the tray icon, Preferences, Uncheck Use TailScale DNS

This is really difficult to diagnose and eventually required a Packet Capture, although it may be in some Exchange365 log as well.

Classic example where Understanding the problem is WAAAAY more difficult than actually fixing the problem.