For a few months all emails I have received from one particular contact have been automatically marked as Junk. When I click Not Junk, I will return later to find it has been re-flagged as junk. Sometimes it will get re-flagged seconds after I mark it Not Junk - this is very frustrating as these are important business emails. Server type is IMAP, I have checked my mail using the provider's own web-based mail client and the messages are not marked as junk there. I have added the contact to my address book and instructed Thunderbird not to flag messages from contacts in my address book as junk, but it's still flagging them as junk, and re-flagging them when I try to unflag them. How can I fix this?