Internet connectivity on 3g iPad when on a non-internet-connected wifi
When connected to both Wi-Fi and 3G, you have two IP addresses. One handed out by DHCP on your portable Wi-Fi network and one from AT&amp;T (or Verizon, as may be the case). 

To my knowledge and from my experience, your iPad will use the IP address obtained by Wi-Fi first, and then 3G. So if you're connected to a Wi-Fi network that doesn't have internet connectivity, the email address you're trying to check has nowhere to go since you don't have internet service connected to your portable Wi-Fi network. 

Unfortunately, this means that in order to check your email, you'll have to disconnect from the portable Wi-Fi network, allowing the 3G IP address to become active.