I just switched from Windows 8.1 to Windows 10 a few months ago.
I installed Windows Enterprise 10.0.19045.2604 and Pegasus Mail 4.73.639, Mar 30 2018, build 639, later I switched to Pegasus Mail 4.80.1028, Jan 16 2022, build 1028.
The problem is always the same: I can receive and send emails, but not emails with a larger attachment (approx. > 10 MB). If I try this, Pegasus Mail starts sending and at some point at 27% or 75% (depending on the size of the mail, numbers are only examples) it breaks off. Much worser: The entire Internet connection (RJ45-LAN via cable modem) breaks down. To get access to the Internet again, I have to unplug the rj45 connector and plug it back in.
At first I suspected my email provider, but an email address from a completely different provider shows the same behavior.
I also tried all kind of secure connections, STARTTLS, SSL etc., without success.

What the heck is this? In the meantime I've switched to Thunderbird, which actually is a piece of crap.....

One thing for certain is that Pegasua Mail won't affect your LAN connection. The only thing that I think might be contribute to the problem from the Pegasus Mail side is that the internet connection timeout is too short allowing the connection to timeout before the send finishes. I suggest setting SMTP the timeout to 120 and see if that makes a difference.

Beyond that, I don't know why Thunderbird would work and Pegasus Mail won't. I assume you have duplicated the working SMTP settings in TB to Pegasus Mail.

Have you tried tools / internet options / create internt session logs.
Then try the connection, and afterwards examine the smtp log in TCPLogs subdirectory.
Perhaps it may show some kind of message??