Setting Email Accounts to Only use 3G

Hiya

At work we have MS Exchange Server set up and my iPhone is set to do the full Calendar, Contact and Mail sync from that server, catch is i can only use it on my phone when im accessing the server from an external IP address eg 3G. Does anyone know of a program that will let me choose what connection each email account is to use when checking for new mail? Im jailbroken so options are open there as well.

Thanks

Nothing I'm aware of. Why not simply set that account to check manually?

That kinda takes away the whole point of push activesync though. Plus I'd have to be manually checking while I'm not in the office to see if I've received any emails. It's not a major but it's just one of those things that's kinda annoying if ya know what I mean

If you're talking about exchange mail - notifications are pushed to you it's not your phone going off to check so setting up how your email connects, if you could do it, wouldn't make a difference.
How is the problem presenting itself? It it that it doesn't work on wifi?

When im in the office we have a MS Exchange server set up where basically because all the iPhones are set to use the external address to connect through to the exchange server, the only way to access the exchange server while connected through Wi-Fi is to change the server address to the name of the exchange server eg. exchange.server.lan instead of eg. exchange.office.com or to switch the wireless off and use just a 3G Network. When im on the wireless it just gives me an error message telling me it cannot connect to the server, because im trying to access the exchange through an external address on the internal network. Gmail works fine through both the 3G and the Wifi

Thanks