I've just come off the phone to Virgin Mobile customer service - it's just as well it's a flat rate of 10p a call, because it took 24 minutes of pretty awful music (VM really needs to get its music server fixed - there's horrible static in there, and the promised press-1-to-change-track feature didn't work) to get through to a human, and nearly 40 minutes before we'd finished!
I called to get my access to the VM website re-enabled as I'd stupidly got the password wrong (it's got to be all in capitals, FFS), then as we were finishing up that she asked was there anything else she could help with...
I remembered that I'd tried using my trusty Palm Tungsten T3, Bluetooth and GPRS to access my O2 e-mail but it had resolutely failed every time, even though Web access worked, as did some other email accounts. So I asked if the O2 account was being blocked in the network.
After I'd explained that no, it wasn't a handset problem - I was just using the phone (a SonyEricsson T630, since you ask, reflashed with the latest unbranded firmware) as a modem, and that yes, it worked fine on other data services, the agent (who was friendly, patient, and as helpful as she could be under the circumstances) went off to talk to the technical department.
The answer, when it came, surprised me somewhat: Virgin Mobile does not officially support mobile email access. Apparently it might work, depending on my handset, but it might not. Again, I explained that the phone is only a modem and that this is a network issue, but to no avail. VM might launch its own mobile email one day, she said, which will be supported, but until then we're on our own.