To find out more about mail, mailx, and Mail, see Chapter 2 or consult your system man pages.
Chapter 17 of this book covers sendmail and other advanced e-mail topics.
There is a lot of information about using e-mail available on the web. In particular, you can find out more about the primary e-mail clients discussed in this chapter at these sites:
Pine Information Center, http://www.washington.edu/pine/
Gopi Sundaram’s comp.mail.pine FAQ, http://www.zrox.net/Mail/Pine/
The Mutt E-Mail Client, http://www.mutt.org/
The Mutt FAQ, http://www.fefe.de/muttfaq/faq.html
Thunderbird, http://www.mozilla.com/thunderbird/
KMail, http://kmail.kde.org/
The KMail Handbook, http://docs.kde.org/development/en/kdepim/kmail/
Evolution, http://www.gnome.org/projects/evolution/