I've been using Eudora for ages. Very few complaints, and zero virus worries (as long as one doesn't do something dumb like set MS Word to be one's email viewer, or go run attachments deliberately).
I use Versions 2.2 through 4.2. Later Versions start to add annoying "pay, or receive banner ads, or sent usage statistics to Qualcomm", so I just stick with earlier Versions, but if you don't mind the other stuff, the later Versions are otherwise fine AFAIK.