I set up an application specific password for Gmail (required because I have two-step verification enabled for my Gmail account), and the password works for Calibre, but the next day, it fails and I have to revoke my password and create a new one. I can tell from the error message that the password is the issue.
Traceback (most recent call last):
File "site-packages\calibre\gui2\email.py", line 43, in run
File "site-packages\calibre\gui2\email.py", line 130, in sendmail
File "site-packages\calibre\utils\smtp.py", line 135, in sendmail
File "site-packages\calibre\utils\smtplib.py", line 617, in login
SMTPAuthenticationError: (535, '5.7.8 Username and Password not accepted. Learn more at\n5.7.8
https://support.google.com/mail/answer/14257 t133sm4817360ywe.54 - gsmtp')
So again, I do a new password, and it works for a day, then fails. My computer remains on with Calibre open (as I have it as a server).