I have an IPhone 4 that constantly has trouble getting e-mail to download. I will pull down on the app and it will state checking mail, sometimes it will indicate 1 of xx emails downloading, but all of a sudden it will abort and state updated as of XX/XX. other times I pull down on the app and it won't even say checking email, it will just already give the updated status. every now and then it will update, but I have to pull down on the app no less then 20 times, usually more to get it to finally update and most of the time it will only update when connected to my home Wi-Fi, it won't update when connected to the 3G cellular network. I constantly reboot the IPhone to no avail.
Did you restrict email to load over wifi only?
it is possible your 3G coverage is not enough to load email.
you can try deleting your email account and adding it back.
if you are using the built in mail app, try downloading a different app. I use gmail and yahoo mail app because the built in app is terrible.