I personally always felt that "full applications" were disconnected from the Unix philosophy. By definition, such an application does many things: fetch things from the network, display them, sort them, ... it talks many protocols, concerns itself with many I/O systems.
I'm certainly not saying that it is the way to go, but a real Unix philosophy way of dealing with email would be to use a command to pull mails from a mail server as text files (heck, as a text stream!), then other commands to display them, sort them, ...