Just to add a few of my own notes when I ran into this problem and finally resovled it earlier today -
I was having the problem between different SSID's. One public SSID was having no issues accessing emails, yet an internal SSID was unable to reach any email services outside of our internal exchange. What I determined was this:
Each SSID was using a different Role when connected. Each role has a different firewall policy that can be assigned. So I first had to find the missing rules and fix this issue. I added in TCP/993 into the stateful firewall at the system level, which to my understanding filtered it down to the "Authenticated" role which is set to allow all. Second, I had to set to allow TCP/993 through my corporate ASA.
Only after performing both did the emails begin to flow again on this internal SSID.