A gateway would be no good for me I fear. I have my own mail server, so I can choose whether to connect to it with POP3 or IMAP, so I have no problem getting my email with Agent. I want to move to IMAP though, so I can keep my mail on the server in organised folders and therefore access it sensibly from several locations.

After reading this thread I have gone back and tried the latest version of Mulbery again (I have tried it out before) and it looks like it might actually now be able to thread like Agent. I can't work out how to move the order of columns in the mailbox view yet (it displays Author, Date, Subject where I want Subject, Author, Date).