kmail problem

I have used kmail 5.9.3 (Pop3) on my Debian Buster system for years.  It is working well 
on 3 of our computers, all of which use the same server. However kmail simply won't 
work on one computer.  It first quit sending and now won't receive either.  As I can it uses 
the server's webmail service just fine, those folks just tell me the problem is on my end.

The error generated when receiving is "unable to log onto server"  "authentication failed"

The error generated when sending is about the same "failed to transport message."

If there is anyone who could help guide me out of this horrible mess, I would be most 
appreciative.  I would be happy to contact you off line if preferable.  

As its her computer, and she relies heavily on kmail, my secretart went home rather than 
just sit around waiting for her kmail to be fixed.

Thanks

John White