Nigel,
Is it safe to assume from the wording of your post that it does receive mail on 3G?
The reason I ask is my Roadrunner account won't accept outgoing mail to it's server that doesn't go through a Roadrunner connection (Wi-Fi).
I now have it going through a new "mobile" server they set up, and it works fine.
If you by chance have Roadrunner mail, I can give you the particulars.
If you have another provider, you will need to google their "mobile outgoing server settings".
Of course; if you cannot receive mail, it's probably a different setting.
Details and particulars will be important.