I just logged in to the forum to see if anyone else had this problem. I've noticed it today for the first time while sending via outlook.com (which Calibre labels as "Hotmail").
I've checked the settings against the SMTP settings on
this Microsoft page but when I test the settings in Calibre it reports "Authentication Failed".
The same details are used from my Android phone and my iPad to send emails via Outlook.com, but only Calibre is having this problem.
I've looked through the commits to the code-base on GitHub and there don't seem to be any recent code changes which mention email functionality, but if it''s not a Calibre-related change then why can other devices still send email and Calibre can't?