Both Gmail and Outlook save a copy of outgoing SMTP mail in the users sent mail folder as the mail is processed by their SMTP server. With IMAP mail accounts that are correctly configured to use a single sent folder synced to the IMAP server this leads to duplication of sent mail in the sent folder. As there is no Optout in the account settings for this action by the host, I suggest we stop making copies of sent mail and placing it in the sent folder for these two providers.

Manuel workaround: Settings --> Copies & Folders --> Unselect 'Place a copy in:' under the "When sending messages, automatically:" heading. However, agree with Matt that the manual workaround should be the default setting when setting up IMAP accounts, like Gmail, Hotmail, Zoho, etc. Who should be cc'ed into this ticket who can implement this behavior? Thank you

May also need Thunderbird to refresh Sent folder once email is sent so that mail will be available in Gmail.

(In reply to Óvári from comment #1) > However, agree with Matt that the manual workaround should be the default > setting when setting up IMAP accounts, like Gmail, Hotmail, Zoho, etc. General IMAP servers don't automatically save sent mail, so you'd need to special case Gmail and Hotmail.

(In reply to Jorg K (GMT+1) [Very limited availability 18-22 Jan 2018] from comment #3) > (In reply to Óvári from comment #1) > > However, agree with Matt that the manual workaround should be the default > > setting when setting up IMAP accounts, like Gmail, Hotmail, Zoho, etc. > General IMAP servers don't automatically save sent mail, so you'd need to > special case Gmail and Hotmail. Please add Zoho to the special case with Gmail and Hotmail. It would help remove confusion with new Thunderbird users as they won't have doubles of messages in their Sent folders.

The gmail case is covered in bug 1427619