Gmail automatically saves copies of all messages sent through its SMTP servers into the sender's Sent Mail folder. As such, it is at best a waste for Thunderbird to save copies of sent messages there -- it makes the sending process taking longer, and requires large messages to be uploaded twice, once through SMTP and once through IMAP. Worse, it can actually cause problems with the sending process. Sometimes when Thunderbird tries to save a copy of the message into Sent Mail at the same time as Gmail is doing it automatically, Gmail's IMAP server returns an error to Thunderbird, which causes an error pop-up. At least, that used to happen in the past, though perhaps they've fixed it. Fortunately, Gmail automatically dedupes messages, which means the user doesn't end up with two copies of each sent message in Sent Mail. Still, the automatic Gmail configuration that gets created in Thunderbird when the user adds a Gmail account should have saving copies of sent messages disabled, for the reasons outlined above.

> Sometimes when Thunderbird tries to save a copy of the message into Sent Mail at the same time as Gmail is doing it > automatically, Gmail's IMAP server returns an error to Thunderbird, which causes an error pop-up Interesting issue. Can you remember some of the text of said error message? Some potentially related bug reports - Bug 507200 - "Unable to copy message to sent mail folder" occurs frequently in Gmail IMAP ("IMAP server directory: [Gmail]" is intentionally set by user) - duped to Bug 413240 - Save to Sent was successful, but "Copying to sent folder" doesn't finish, or zombie "Copy complete" - Bug 543232 - "Replied" icon not appearing in message list when saving to Sent folder fails on Gmail IMAP, but msg was sent - Bug 550407 - Gmail doesn't add Gmail Label of Inbox to arrived mail, if the mail is sent via non Gmail SMTP by To:/CC:/BCC: and if Gmail IMAP folder is used for "folder to save sent mail copy"

tangential Bug 589303 - BCC-recipients are not displayed to me in sent mails which is saved in [Gmail]/Sent Mail by Gmail's STMP server

It's been too long since I've seen this error myself to know exactly what it is, but I suspect Bug 507200 is probably it. The BCC issue is a problem, but I'm not sure there's anything to do about it... As far as I know it's impossible to turn off Gmail's automatic saving of sent messages into the Sent Mail folder, and if Gmail saves the message and then Thunderbird does, there's no way of controlling whicho f the two copies of the message gets kept by Gmail.