Every time I run Thunderbird, it appears to lose its IMAP connection(s) after 10-15 minutes. I've tried varying the number of cached connections from 1 to 10 with no difference in behavior. Restarting Thunderbird will restore connectivity temporarily, until the entire process starts over again. It seems to be correlated to saving a copy of sent messages. The sending process times out while the message is being saved to the Sent folder after the SMTP sending is complete. Once this happens, new mail doesn't appear, moving mail between folders has no effect, and some (but not all) messages can't be retrieved from the server.