I would suggest Kayako E-Support. It's more than a mail manager, but if you are serious about documenting emails, merging conversation histories, allowing multiple operators to respond to discussions etc, it can't be beat.
And the web accessible interface allows folks to contact you from a computer without their own ISP email access.
As a free option, you might want to look at Cerberus Helpdesk.
http://www.cerberusweb.com/
A single email address licence is free. Multiple emails costs money...