Mail lost a 'sent' message - why?
I recently sent a mail (IMAP) from my .mac address to a person I often email. Got the 'swoosh' etc. A couple of days later, I wanted to check up on what I'd said, so I looked in 'Sent', only to find no trace of that individual mail - there were a lot of other messages in 'Sent'. I began to doubt if my mail had indeed been sent, but then my correspondent replied, complete with my original mail as a quotation! So the send worked, but the sent mail was mysteriously deleted (and no, it wasn't in the mail trash either). What could possibly have caused this? I don't know any way of avoiding the 'Sent' mailbox, so I can't imagine what mechanism could have caused it. Has anyone any ideas? I am scared that this may happen again - I rely heavily on the 'Sent' mailbox to remind me of stuff I've written.
iMac, OS X Yosemite (10.10.5), New for 2012. 8Gb