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Fri, Jul 22, 2016 5:56 pm

Troubleshooting POP3S connections through a NetScreen firewall

A user reported that she was unable to receive email using her Outlook email client. When I checked her system, I found that Outlook 2016 was reporting the following error message:

Task ' - Receiving' reported error (0x80042108): 'Outlook cannot connect to your incoming (POP3) email server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'

I verified that I could connect to the email server on the SMTP port, port 25, and the POP3 port, port 110, from external systems and she told me that her husband's system was able to check his email successfully. Since I knew his system was establishing a POP3 connection to the mail server to check email, but her system was establishing an encrypted POP3S connection on port 995, I checked the Linux email server to verify that it was listening for connections on port 995, since there had been a power outage at its location recently that I thought might have caused it to reboot. It was listening on that port.

$ netstat -a | grep -i pop3s
tcp        0      0 *               LISTEN     
tcp6       0      0 [::]:pop3s              [::]:*                  LISTEN

So I thought the problem was likely at a NetScreen firewall between the user and the server. I logged into the firewall via Secure Shell (SSH). I verified that there were no filters currently set for the firewall and then set a filter for destination port 995 with the command set ffilter dst-port 995. I then cleared the debug buffer with clear db and started debugging on the router/firewall with debug flow basic - see NetScreen Debugging Commands for an explanation of the debugging commands. I could see that TCP packets were reaching the firewall, but the firewall was reporting "packet dropped: for self but not interested".

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