I use MS Outlook mostly as it is very easy to set up with networks. A few years ago I used Euroda (not entirely certain bout the name), but it was a pain in the ass when you wanted to use it in networks!
I´ve been using Eudora for quite a few years now. It offers a lot of functionality and comes in free, sponsored or commercial versions. It´s also available for both Windows and MacOS. Pretty good, all in all.