having same problem
I have two email accounts set up. My "push" yahoo account works great. Not 100%, but I'm often pleasantly surprised at the speed. I'll delete an email on my computer and it will be reflected almost immediately on my iphone. Very cool.
However, on my work pop account I setup, the iphone says its a valid account. But when I go to connect to get emails, it acts like it's getting them, but none list even though I know there are emails to read, even new unread ones. The same settings used to work on my Treo 650 (the iphone blows it away, btw) so I'm not sure why they aren't anymore. Tried flipping authentication settings but it always switches back to NTLM. SSL is off as it should be. The server is correct. My username and password are correct. Just don't know what else to do...