It’s possible that your work has problems with spam and has thus set up their own spam filter and that filter might be more aggressive.
There are also ways to change the spam filtering in m365 mail.
It’s also possible that your work hasn’t setup mail properly and protonmail is the one that rejects the mails. We had an issue at work where OOF auto replies didn’t work to external gmails (and probably others) because Gmail rejected the emails (I had confirmed that Gmail was the one that rejected it via the email traces in exchange). That wasn’t Microsoft’s fault.
I can send stuff to my proton mail without issues from two different Microsoft 365 mails.
One missed email should not be interpreted as malicious intent. It’s also honestly quite likely someone just misspelled or something.
Walking very close to someone’s home is also illegal with the right to roam. The right to roam just gives everyone the right to walk were they want except in someone’s garden. You can also camp anywhere (gardens excluded of course) for a day (or two can’t remember) without asking anyone for permission.
One kinda surprising thing is that everyone is allowed to enter fenced animal pastures, provided that they aren’t malicious and that they close the gate.
It’s an amazing right that should exist in the entire world.
The European Union is a union and not a country, the EU is pretty irrelevant here since there are lots of Europeans that don’t live in a member country. There are lots of cultural differences and languages.
Of course everyone in Europe is European but generalising Europeans that really doesn’t have that much in common (except living in Europe) is most often pretty stupid. Europe is very diverse in culture, climate, economy, politics, etc.
Do syncthing support version control? I currently use Git with GitHub to sync between devices and while it works it can be incredibly annoying to troubleshoot when it goes wrong.