Is anyone aware of any? I'm specifically looking for any that are well maintained (in the Ruby community that generally means the source is availble on GitHub). Here's what I've found in my research:
MailCatcher runs a super simple SMTP server which catches any message sent to it to display in a web interface.
Sup is a console-based email client for people with a lot of email. It supports tagging, very fast full-text search, automatic contact- list management, custom code insertion via a hook system, and more. If you're the type of person who treats email as an extension of your long-term memory, Sup is for you.