3g cant send emails as all are "rejected by server"

Hiya
Love my Pro and iPad but the phone, 3rd in 14months, is driving me mad.
Only had this replacement 3weeks and now cant send any emails. They all end up in my outbox with the "rejected by server" window. Have wiped and restored all settings - but no joy. Can anyone help?

When I get this it usually means my iPhone's email out-going server is wrongly configured, for example it's set to my home network provider's server when I'm in my friend's house or at work.

Go to: Settings -> Mail, contacts, calendars -> your mail server

Then scroll down to: Outgoing Mail Server

Add and switch ON the one nearby and switch OFF any you don't need.

You're supposed to be able to keep more than one of them ON at once but I fond this fails and I keep having to go and turn off all but the one I'm near.