I use the mail client that comes with mozilla and im very sattisfied with it ..
if you prefer a standalone version you could use thunderbird as said above
www.mozilla.org
"I looked at KDE Kontact and it offered promise but it only allows you to have one contact folder. I prefer to operate several eg Business, Private, Club etc.
I also want ability to mass mail for small groups of contacts ...."
The mail client i use has this features as allows to have multiple adress books of contacts and also is able to mass mail predefned groups...
Is able to filter mail and put it in correct folders so for my needs is perfect but as said above it depends to much of one use it .