Sometimes the iPhone’s works get gummed up and the phone isn’t able to download email. On phones running firmware version 2.0 or higher, this seems to happen most often after a charge, but there’s no apparent rhyme or reason to the problem.
It’s a particularly frustrating problem in that in almost all cases, Internet access works just fine for other programs.
Luckily, the solution is an easy one: just restart your phone. After it restarts, email deliver should return to normal.