Heirloom mailx (formerly known as "nail") is derived from Berkeley
Mail and provides the functionality of the System V and POSIX mailx
commands. Additional features include support for MIME, IMAP
(including caching and disconnected use), POP3, SMTP, S/MIME,
international character sets, maildir folders, message threading,
powerful search methods, scoring, and a Bayesian junk mail filter.
Mailx can be used as a mail batch language in nearly the same way as
it is used interactively. It can thus act as a mailbox filter, can
fetch mail from remote accounts, and can send files as attachments.

WWW: http://heirloom.sourceforge.net/mailx.html