presumably you're not "receiving" emails here, you're going and fetching them, right? so check if you can telnet to the imap or pop3 service you are trying to reach. TBH it's really GOT to be a network issue, or potentially DNS if you need to use a proxy for normal web traffic.
telnet mail.mymailhost.com 143
telnet mail.mymailhost.com 110
for standard imap and pop3 respectively, and see what that does.