Secondly, mail, mailx and sendmail are very different beasts. The first two are mail clients, they read whats in your mailbox and can send mail to the MTA (mail transfer agent). Sendmail is a SMTP server (I won't go into why it shouldn't be used). SMTP - simple mail transfer protocol. The SMTP server is the thing that does the work, delivering mail so your client can read it.

There is shell access to sendmail but it isn't pretty.

You can read/parse mail headers with Perl, C and other languages. For Perl modules see the CPAN link above.

CC

The above is an over simplification of how mail works.