[Users] Claws Mail Can't Connect to Internet With Specific Internet Connection

[Dustin Miller]

Claws Mail (CM) 4.1.0 on Linux Mint 20.1

My system has access to two different internet connections (IC):

IC #1: Via LAN cable to a local network that has internet access.

IC #2: Via a USB dongle wireless device using a SIM card to connect
with a mobile network.

Starting a week or two ago, when using IC #1, I can use the terminal to
ping 8.8.8.8, my browser works fine, but CM cannot access the internet
for sending / receiving email. When trying to check email, only part of
the dialogue window shows up at first, then later the whole dialogue,
then eventually a timeout error window. Network log just shows the
timeout error. Problem affects Gmail (POP3), Outlook (POP3), and MS
Exchange (IMAP) accounts. No Claws Mail settings were changed or
updates done previous to the problem starting. Even though there is no
functional internet access, CM seems to 'think' it is connected (i.e.
the 'online / offline' icon is connected / closed, which presumably (in
this case) just means that Network Manager realizes it's connected to
the local network. I haven't come across any other programs I regularly
use that have this same problem connecting to the internet.

When I disable IC #1 and connect IC #2, then CM and everything else can
access the internet as expected.

I'm reasonably certain that this isn't a CM problem, and am suspecting
it is some sort of a DNS / nameserver issue with my system and IC #1.
(BTW, I also confirmed with the IT person managing IC #1 that they
haven't changed any settings on any of the relevant network devices
that could be affecting this.) I guess it's also possible that IC #1's
ISP has changed settings (port-blocking?) that could be affecting this.

In any case, I am wondering whether anyone (probably a developer) can
give me a basic idea of what is supposed to happen (i.e.
'behind-the-scenes') in the interaction between CM and its host system
(Linux-based) when a person directs CM to check email? Or perhaps point
me to a resource or documentation where I could find this out myself?
Might be a long shot, but I thought I'd check in here to see if I can
get some info that will help me know how to move forward with
trouble-shooting this.

TIA,
Dustin