Recently I came across a situation in a company which ran ISA Firewall where the Outlook clients were not able to connect to external POP3/SMTP servers. The implementation of the firewall was being done by one of my friends and he was stuck up with this problem.
Upon further discussions with him, I came to know that the clients were using the ISA Firewall client. The ISA Firewall machine was not a member of the domain – which is a good sign. There was a rule configured which allowed DNS, POP3 and SMTP protocols from the Internal network to the External networks. The rule was enabled for all Authenticated Users.
So far so good. Everything seems to be in place and configured right. But what is stopping this traffic?
The problem was the default Firewall Client settings. In the application settings of the Firewall Client settings, OUTLOOK was set to Disable. Modified the value to 0, refreshed the Firewall Client and attempted a connection. BINGO! Everything was working fine now and a treat from my friend was due.