re :I have no rules set up in Thunderbird to automatically move any email anywhere.

You may not have any Message Filters, but you may have the Account Setting > Junk Settings set up to move Spam/Junk to Trash ? In those Junk Settings, you may also have selected the checkbox to 'Trust junk mail headers set by' a server spam filter. Thunderbird may be following those instructions.

Also have you set up any whitelists for address books? Are those email addresses included in one of those whitelisted address books ?

Are the 'Global Junk Settings' set up to Delete emails marked as junk?

• My Junk Destination setting for the account is set to move new junk messages to my Junk folder, not trash.

• "Trust junk mail headers..." isn't checked.

• I have nothing checked under global Junk settings.

I don't have adaptive junk mail controls set up. I've been letting the server-side algorithm handle that. Some junk does still get through, so I like the idea of using the adaptive controls to teach Thunderbird and will probably enable it.

While the whitelist will be useful for me in terms of adding these addresses, that only (potentially) works if I know an email address has been affected by this peculiar behavior. It's useful, but it doesn't keep it from happening altogether.

The annoying thing about this bug is I can only confirm it if I get email from either of these accounts/domains again, which I can't control. Maybe I'll find a way for Apple to re-send a notification. :)

Thanks a lot for your time!

I think more to the point, what anti virus do you have trying to work out what is spam and what is ham and what is a scam. Most, if not all of the folks I have seen with this issue were getting unwanted assistance from a spam tool in their internet security package.