I was with o2 Pay & Go, and wanted to use email (I moved from Contract to Pay & Go so couldn't work out why it didn't work)
Anyway, I contacted them, and they told me the secure connection needed for email is not supported on the Pay & Go tariffs, so if I needed email, I would have to change to a Pay Monthly...
Which I did, and it's been fine ever since.
Nokia History: 3110, 5110, 7110, 7110, 3510i, 6210, 6310i, 5210, 6100, 6610, 7250, 7250i, 6650, 6230, 6230i, 6260, N70, N70, 5300, N95, N95, E71, E72
Current Phone: HTC Sensation - Android 2.3