Many times when I have finished writing an E-mail and click to send it, an authentication window appears with my username and an empty password field. This is despite my authentication information being saved, and appearing in the Account Configuration panel for the selected SMTP account. When I click cancel, then select File --> Send Queued Messages, it successfully sends the message without prompting for my authentication information. All future E-mails composed and sent from that SMTP account are then sent successfully without prompting for authentication information.

Reproducible: Sometimes