First email send by smtp is lost (!) when OAuth2 is used for authentication and token has expired.
When the token for sending an email with smtp and OAuth2 as authentication method has expired, there is no warning message to renew your credentials, but the send email is lost. When sending a second message directly after the first attempt, the window to renew your credentials pops up for the first time and this message will be send normally. This is quite annoying, since one assumes, that the first message has been send.
(Thunderbird 45.5.1, Ubuntu 14.04)
Wšykne wótegrona (1)
what email provider do we use oAuth2.0 for SMTP on?
However sounds like a valid bug. Please file it in bugzilla. https://bugzilla.mozilla.org/