That sounds like your providers retention policy. Remember your email in IMAP account is synchronized to their server, so if they remove mail from the account for any reason, it will reflect in Thunderbird's copy.

Thanks Matt, It's odd that this problem only occurs on one account despite there being three others with identical settings. Should I perhaps try POP do you think? Brian

What does you provider have to say about retention policies? You have not mentioned who your provider is, so It is not like I can go exploring on my own.