I actually haven’t tried DavMail (but have heard about it), so if it manages to get around this sort of shenanigans I’ll happily give that a shot.

In my previous org I could also use offlineimap and msmtp to connect to their Microsoft mail server via standard protocols. But in this org I’ve so far tried the built-in Exchange support in Thunderbird as well as in Evolution Data Server based exchange clients (Evolution and KMail). All of them manage to connect to the server, kinda, but then I get an error message saying basically that my mail client is not approved and I’ll have to contact my admin to use it.

EDIT: I might add that the IT deliberately blocked non-Outlook mail clients a year ago or so, other Linux users told me that it worked fine before that. It’s supposedly a crackdown on people using shady third-party apps that they are concerned might exfiltrate data, but somehow they don’t allow exceptions even for reputable clients like Thunderbird.