Mutt
True to the motto "All mail clients suck. This one just sucks less", mutt is a very fast and powerful text-based email client for real systems [for others this might or might not help]. 
I use Mutt for reading all Internet mail, and most of the mail I send to Internet correspondents also originates in Mutt, written with this of course. Mutt is first when it comes to testing new systems, as it allows me to quickly and effectively see headers, bounce messages or simply delete them en masse (think: Spam).
Mutt is highly configurable and allows its users to do almost everything with a bit of mail. Its documentation is quite good (I recommend the MuttWiki), and the Mutt Primer can get you started quickly if necessary.
Oh, and if you need some inspiration on getting Mutt's configuration file .muttrc filled, visit the muttrc builder site.
Jan-Piet Mens / Mutt as Anti-Spam and Anti-Malware Tool wrote on 16-Apr-07 at 11:44 pm
[...] mutt can protect you from a huge amount of rubbish that lands in your [...]
Jan-Piet Mens / Replace mail by nail (Heirloom mailx) wrote on 07-May-07 at 11:57 pm
[...] mailx would be much too rudimentary for me to use as an everyday email client (I use mutt for that), but as a CLI tool for embedding into scripts or programs, nail is the best [...]
Jan-Piet Mens / Using Domino Mail from Unix/Linux wrote on 03-Aug-07 at 11:49 am
[...] be using Mutt in these samples, but almost any standards–compliant client be it graphical or [...]
Jan-Piet Mens / Namazu: a Full-Text Search Engine wrote on 20-Aug-07 at 1:08 pm
[...] folders via mknmz. The beauty of this tool is the integration into mail readers such as Mutt to produce a very fast index that can be searched for within the [...]