Gregory's Server
It would depend on which piece is verifying the SSL/TLS certs.
If the Outlook (New) client does the "I'm talking to neilscorp.example.com" verification, then it's less bad because in theory the MS machines are only acting as a conduit for encrypted bits (still dumb because it breaks when attempting to connect to mailservers in RFC1918 ranges).
However, if the local client is talking to the MS cloud, and it's the cloud machines initiate the conversation to the mail-server and checking the SSL/TLS certs (which is what I suspect is happening; @nielsk doesn't provide pcaps nor detail whether the client is connecting to some web-servicey port like 443 instead of mail-servicey ports), then SSL/TLS certs don't protect.
@ed1conf @UkiahSmith The mail client talks to the MS cloud and the MS cloud talks with the mail servers. Tbh I only looked at contacted IPs and didn’t dive deeper because I just wanted to see why it didn’t work for my colleague. So I looked at the dump and searched for the IPs I expect them to connect which they didn’t and looked at my mail server logs and only saw an MS cloud-IP. (1/4)