email

If you are not receiving mail on your iPhone, then something else is going wrong. Before you delete the mail account, log into your mail on a web browser and check to see if your mail appears there. If it does, then go ahead and delete the account. Power the iPhone off and back on again. Then add the account back. Take your time and make sure that the account goes through the verification process so you are sure everything is connected.

I would also be curious as to what kind of error message you received when you opened mail and the client attempted to connect.

That would depend on why the emails are missing and what type of account this is, i.e. POP, IMAP, etc.. You need to provide more context.