Issues sending/receiving emails using mail.domain.com - Mobile

[Injection_Mold]

Not sure if I have something configured wrong or I have posted this in the wrong category. I am new to PFSense so please bear with me. When mobile users are connected to mobile network they can use active sync with mail.domain.com to send/receive emails. When they connect to the internal wireless they cannot send or receive emails using mail.domain.com. This worked fine until we added the PFSense firewall. What do I need to do to get this to work? Thank you for your help.