The Express Lane is indeed the way to go for things that just won't download from the App Store.
If you're on a wireless connection, please try a wired one when downloading as a wired connection is historically faster and more stable due to the nature of wireless internet.
What do you mean with "unable to upload"?
POP uses TCP port 110 for a non-secure connection and TCP port 995 for a secure connection, make sure the server is reachable on those ports, i.e. by telnetting. I'd use IMAPS instead of POP3, that is the secure version of IMAP and works on TCP port 993, so the users would have all their mail anywhere they go.
It wouldn't hurt to try the Automatic Setting I guess, if it were only to test if the client is able to connect.