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.....