Has the forwarding been set up through webmail access?

No, I haven't done that.

If Thunderbird is closed the emails are still forwarded to the other email address? Your wife receive the email with the subject starting with "Fwd" or she receive the "original" subject (w/o Fwd)?

The messages are all forwarded from Thunderbird when it is opened. The subject lines all start with Fwd:

For Thunderbird to forward to another email address there would need to be a Message Filter set up to auto forward emails to another email address. This would only occur if Thunderbird was running, so if Thunderbird is not running and emails are getting forwarded, then it's not Thunderbird.

Have you enabled Filter Logging ? Menu app icon > Tools > Message Filters Filters for : Select your account Click on 'Filter Log' This log will show you what the filters have done, so you can see which filter is causing the problem. If it is not enabled - select checkbox 'Enable the Filter Log', so you can see what happens when new mail comes into the account.

Do you have any pop accounts that are set up as deferred accounts (Global Inbox) so you use 'Local Folders' account ? If yes, then also check to see if there are any filters for 'Local Folders'.

Check: do you have another email client running in the background on your computer - have you ever used Outlook or Mail etc. I have come across someone who had used another email client in the past and was not aware that it was still running in the background.

Here's what happened. I initially set up filters for a select group of messages (bank statements, electricity bill, etc) to be forwarded to my wife's email account - and that was all. Suddenly she started receiving every message that came to my inbox. The filter log showed nothing amiss. I subsequently removed the filters I had set up and it stopped happening. So far the issue hasn't recurred. Thank you to the posters who responded, but I still don't know what caused that problem.