I've connected a few iphones to the MS SBS 2003 mail system and the problem is with all of them. When I leave the office (get off the wifi) the mail sync's very well both on my home wifi or 3G. As soon as I'm within the wifi of the office I continue to get a message on the iphone that states "Cannot get Mail, the connection to the server failed".
I do get mail sometimes but not always and it hardly ever seems to send when I'm within the wifi.
I have three mail accounts on the phone, they all seem to receive mail randomly while in the wifi zone.