Office 365: Duplicate Items In Sent Folder
If an Office 365 mailbox is configured in a mail client via IMAP, there may be two copies of each sent message in the Sent Items folder. This is because the IMAP client is saving a copy in the folder, and Office 365 is saving a copy of messages sent via the SMTP server.
To prevent this on Thunderbird:
- Go to Tools → Account Settings.
- Under the approproate account, click Copies & Folders.
- Untick Place a copy in: