With no information I will guess your a yahoo customer (even through ATT) as they are the main mail provider that puts bulk in the subject of POP mails they think are SPAM. For IMAP account the mail is directed to a bulk folder.

Matt, thanks for your prompt reply. I didn't give you enuf info. All my email addresses are @pacbell.net, and the incoming server is pop.att.yahoo.com. But, out of a dozen people who have emailed me since I set up TB (I finally deserted Netscape 7.2 Messenger; I'm a technology dinosaur at heart), this one friend is the only one whose emails arrive marked [Bulk]. Her address is @yahoo.com. In fact, I have 5 email addresses and hers are the only ones on any of my pacbell accounts with that designation, and I get dozens of business and hobby emails daily. Hope this will help you help me. Thanks in advance.

Bob, I still think it is yahoo marking the mail as spam, But if you want to take it all the way. I am going to shake you out of your comfort zone probably, but life is like that sometimes.

Add the troublesome account to Thunderbird again, this time as an IMAP account (you can delete it later) use the server name imap.mail.yahoo.com when adding your account (your user name is I think the full ATT email address. with your att password.

Go to Tools menu (Alt+T) > account settings > server settings and change the retention to leave on server, if you have not already (makes a good backup really).

Now when your friend mails you the mail will appear in both accounts what is of interest is what folder it appears in in the yahoo account

addendum: before adding more accounts, log into webmail and add your friend to your contacts/ address book on the pacball web mail.

Most spam filters respect the address books as being ok senders.