Issue report:
I tried adding a new Mail (Other) account today, and after entering the Incoming and Outgoing mail server hostnames, at the Test configuration stage, the SMTP section failed with “Server not responding”. (The IMAP section tested “OK”.)
Clicking on the small Fix button that showed up afterwards fixed the issue.
However, using the Automatic Setup method, the account could get set up successfully.
INFO: The SMTP server only accepts authentication with STARTTLS connectoins on port 587.