Are your email servers blocking access for certain users? Working form home I have to use Hughesnet for an ISP. Using that network I cannot connect to the inbound server . Can ping from Verizon but not from Hughesnet.
Cannot access the Stonemountainltd inbound server at inmotionhosting via Hughesnet. Cannot connect via telnet, and cannot ping the server either. But I can access the server via my cellphone's Verizon connection. When I set the same cellphone to use my Wifi/Hughesnet, the cellphone also cannot access the server, just like my PCs.
I had a friend ping from a non-Hughesnet domain, and his ping went unanswered also. But pinging from within our StoneMountain office works as expected.