old emails are flagged as new when checking kmail

I have had this problem on my debian system for years.  I check email today 
and get today's email PLUS "unread" emails that were read a week or two ago.  
They are not unread.  I read them two weeks ago.  So I have to laboriously go 
through these old-new emails and mark them as read.

About every 6 months I ping this list to see if anyone has a suggestion.  I 
would appreciate any hints on how to avoid the problem of old, read, emails 
showing up as new, unread emails.

thanks

John