When attempting to check email after installing Mozilla Thunderbird on a Windows 10 system and configuring an email account in Thunderbird, instead of email being downloaded, Thunderbird would show "Connecting to pop3.example.com" and seemed to be indefinitely stuck at that point. When I opened Wireshark to observe the network traffic between the Windows 10 laptop and the email server, I didn't see any traffic whether Thunderbird was configured to use the Post Office Protocol version 3 (POP3) port 110 or the secure version of the protocol POP3S on port 995. I set the Wireshark filter to be
tcp.port eq 110 || tcp.port eq 995. I didn't see any traffic between to/from the server when I changed the filter to look for traffic to/from the IP address of the server with
ip.addr == xxx.xxx.xxx.xxxwhere xxx.xxx.xxx.xxx was the IP address of the server. After puzzling over the matter for awhile, it finally dawned on me that I had prevoiusly configured the Microsoft Edge browser to use a SOCKS proxy server via a Secure Shell (SSH) tunnel set up with PuTTY. I had disconnected the PuTTY connection to the SSH server, but hadn't changed the proxy server settings for the system so I realized the was likely the cause of the problem, though I hadn't expected Thunderbird to use the system proxy server setting by default.