Maybe if Mail worked, this would be an issue.
I spent the whole afternoon yesterday trying to make my sister's Windows 10 to send email using Mail, through my hMailServer, via SSL. IMAP works, but SMTP doesn't - without returning any error - albeit the error reporting in Mail is so luser-oriented it would not be much useful.
Looking at traffic with Wireshark, it does begin the SSL handshake, but then Mail sends an RST. On the server side, the server complains about a "short read" - so something must be wrong in the data. No problems using Thunderbird, Windows Live Mail, Windows 8 Mail and RoundCube with the same server, using the same settings. The fact they dumbed down the client configuration doesn't help either.
Just my sister likes the mail notifications it gets from Mail - and Thunderbird UI would need a good redesign.
It looks not so few people have issues using servers which aren't outlook.com or gmail. The best answer found so far is in this thread: https://answers.microsoft.com/en-us/windows/forum/apps_windows_10-outlook_mail-winpc/windows-10-mail-app-email-stuck-in-outbox/5574d879-0d0d-4cb5-888f-48b3358236a9, look for "EdwinSleepDoc".