Since I do not much about your set up, I have to guess a bit again correct me pls if my assumptions aee wrong:
I assume that the PC you tested this with successfully is on ethernet (cable) not on wireless, so it seems that your wireless clients "somehow" get different TCP/IP settings than the PC on ethernet. The solution therefore is to set up DNS so that for someone on your internal WLAN will resolve to the internal ip address and for someone from the outside the name will resolve to the external ip address.
Most firewalls don't like such "inside->outside->inside" connections. Since this external ip does indeed point to your own firewall, the connection, which just went to the outside through your firewall has to come back in right away through your firewall again. If this scenario correct then a iPad on your company's WLAN will resolve the mail server's name to its external ip address, meaning it will reach out to the internet through your company's firewall. DNS will resolve the name of your mailserver ( or whatever) to the external ip address, regardless whether someone is external (e.g.this Exchange server can also be reached from the outside via a redirection from your firewall to this internal mail server.you have an Exchange server in your internal network.For the time being I assume the following: You will probaby have to tell us a little more about your setup, before we can answer your question.