What I've found is that there are a number of text editors for which mail integration goes far beyond simply composing and sending mail.
On Mainframes running VM/CMS now called z/VM, the mail utilities commonly found will generally be build as a set of macros for the highly programmable XEDIT text editor.
The beauty is that you have the full power of your text editor available during all phases of mail creation and editing.
Does anyone else have some email/editor symbiosis they'd like to share?
GNUEmacs/XEmacs? includes a mail "major mode", allowing you to read and reply to mail from within the editor, using all emacs facilities. Along the same line, they include a mode for reading and responding to Usenet News.
Pico was developed specifically for Pine, a mail/news client.