For my gmail account, "sent mail" was not being saved, it was hanging, trying to save, but wouldn't. Then the same happened to my email from another account (not gmail, one from a domain on namecheap). On thunderbird I repaired the "sent mail" folder in my gmail account (repair folder). Now I have NO messages in that folder; they are all gone. There were 4,000 sent messages that are lost at this point. I have an IMAP setup, and I am hoping there is a way to restore the sent emails to the folder. Also -thunderbird is really slow to get messages. Just another issue that seems alive around its performance right now. Still waiting for a message code for session and it's 15 minutes later- had to grab it off web mail for my gmail account.