Do you mean a Email Client (Receiving/Sending Mail) or an
Mailinglist Manager?
Eudora Pro is a very good email client
http://www.eudora.com
Octeth Mail is an excellent maillist manager
http://www.octeth.com/products/oempro/
Donīt use it for SPAM octeth has very good opt in/Double Opt in
features
DynaMite