E-mail Problem

I have two e-mail accounts set up on my iPhone one pop3 and the other imap. I'm having problems with both accounts. They work for a while and then all the sudden they stop receiving mail. They will not fetch every 15 minutes (like I have it set) and when I go into the mail application it just says "Connecting" and never connects or changes to "Receiving Mail." I can fix it by powering off the phone and powering it on again, however it just happens again after some time (usually less than a couple of hours). Anyone seen this problem or have any ideas on how to fix it?