Can't telnet to port 25 and yet Outlook still manages to send mail. How?

Question

I configured Outlook to use port 25 on my ISP's smtp server. That works fine and I can send email. But if I try to use the server in one of my applications (using JavaMail), I can't connect to the server. Same thing if I try to "manually" telnet to it: I get

telnet: Unable to connect to remote host: Connection refused

How is this possible? And how do I configure my application to be able to send mail?

Edit to add more information:

Wireshark shows that when Outlook tries to send email, it connects to port 25 of the ISP's smtp server. Everything works as expected. When I try 'telnet server 25', the connection never makes it to the server. I see 'Destination unreachable (Communication administratively filtered)', which seems to indicate the firewall at my workplace blocked it. If that's the case, how does the firewall know to let Outlook through?

Answer 1

I would use a network sniffer like Wireshark (free) to see what Outlook does - Wireshark's "Follow TCP Stream" is useful for this.

Answer 2

Bear in mind that Windows can block outbound connections on a per-application basis. Check the Windows Security control panel.

Answer 3

techie007 was on the right track. The anti-virus running on the computer is what was blocking the connection. I'm now running the program from a linux box and that works fine.