As opposed to "push" style ActiveSynch, MobileMe, or Yahoo! iPhone mail, traditional POP or IMAP accounts, like Gmail or ISP mail, needs to "fetch", or check the server on a certain schedule to see if there are new messages.
Reader Mike wrote in pointing us to a thread on the Apple Discussion Forums about "fetch" email being broken with iPhone 2.1. I only fetch mail from Gmail, and Gmail IMAP is a strange and buggy implementation which gives me considerable problems beyond the iPhone, so I can't say whether anything is really broken or not in 2.1.
