First my apologies for the quality of the bug report. I should have noted down the exact error message but I didn't expect to lose an email. :-\
I replied to an email that resided on an IMAP server. When I was done, I send the email. The folder in which the original email was had the "Save copy of outgoing messages to this folder instead of Sent" option set. Since I had that folder (and therefore the connection to the IMAP sever) open, I didn't expect any problems with this. But claws-mail opened an error message which sadly, I cannot recall precisely. Sorry. I registered the error message as something going wrong with the login/authentication, so I thought I had just mistyped the password for the SMTP server but after closing the error message the email composing window was also closed. I checked the local sent folder, but my email was completely gone. I later figured from the log that when claws-mail tried to move the email to the IMAP folder, the IMAP server (dovecot) decided it needed to restart itself because of a clock change.
I understand that this might be difficult to reproduce but perhaps you can check the code to see where an email might get lost under such circumstances. While I do hope that this does not happen again, but if it does, I'll try to note down the exact error message and copy claws-mail's logs to help debugging it.