My friend uses PMail for about 18years, as a mere user .
She gets her mail using POP3 to Pegasus, using the setting "leave on server", to also enable reading her mail from the smartphone. Worked for many years.
Now somehow Pegasus seem to "have lost" the "last read counter value", meaning, when she calls for new headers, it grinds through > 5k mails. The latter is no problem in it self, but these mails represent about 8GB of data, so my suggestion, simply let it read all these mails new and delete the "unread", to get the counter right, is really only a last resort.
TL,DR: Can i, and how, set the value for POP3 last read message #?