Thunderbird and Proxy Server Settings

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.xxx where 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.

In Thunderbird, I clicked hit Alt-T to view the tools menu and then selected Options from that menu.

Wireshark Crash Course
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Thunderbird Tools menu

I then clicked on the Advanced tab in the Options window.

Thunderbird Options window

From the Network & Disk Space option, I clicked on the Settings button to the right of "Configure how Thunderbird connects to the Internet. I then saw that Thunderbird was indeed configured to attempt to use the SOCKS proxy, since configuring the proxy settings through the Microsoft Edge browser and resulted in those settings becoming the system proxy settings.

Connection Settings - use system proxy settings

I clicked on "No proxy" and then the OK button and then OK again at the Options window. Then when I clicked on Thunderbird's Get Messages option, I was prompted for a password for the account, since I hadn't saved it. I also saw that Wireshark was displaying the 3-way handshake used by the Transmission Control Protocol (TCP) to set up network connections between client and server systems. E.g., the laptop sent a SYN packet, the server replied with a SYN ACK packet, and then the laptop acknowledged that packet with a ACK packet. I then saw the normal transactions between a POP3 server and an email client.

So, if you need to change Thunderbird's behavior in regards to whether it does or does not use a proxy server, the steps above can be taken to alter its configuration in regards to use of a proxy server.