Microsoft 365: Duplicate Items In Sent Folder

If an Microsoft 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 Microsoft 365 is saving a copy of messages sent via the SMTP server.

To prevent this on Thunderbird:

1. Go to Tools → Account Settings.

2. Under the approproate account, click Copies & Folders.

3. Untick Place a copy in: