We managed to send to everyone except MSFT.

MSFT sends to us but then they don't accept anything from us, not even bounces to their own mail.

Relay between gmail addresses works fine. (ARC is enabled and working for years? ... since rspamd supports it. [though it had mysterious bugs initially, so I spent quite some time with it.])

> The milter is a total pain to set up, validate, and get working.

yes :D

especially the sender rewriting one.

> High volume sending also has stringent requirements.

yep, and it makes sense! though we're nowhere near that.

thanks again for your detailed replies!