Access mail server from internal lan

Hi everybody

I'm quite new so sorry if it's a dumb question

At work I have a FortiGate 40F and a mail server on a Synology NAS, I have open the ports that I need to connect the server from internet and added all the roules. Now I can connect to it from home with PC and Phone using a FQDN.

The problem is that when I go at work and the Phone connect to the WiFi (internal lan) the mail app on the phone stop to work using the FQDN but I must use the internal IP.

Must I add a new Rule? Where?

Sry again for the dumb questio

Thanks Stefano