The Horde_Mail library is a fork of the PEAR Mail library that provides
additional functionality, including (but not limited to):
* Allows a stream to be passed in.
* Allows raw headertext to be used in the outgoing messages (required for
things like message redirection pursuant to RFC 5322 [3.6.6]).
* Native PHP 5 code.
* PHPUnit test suite.
* Provides more comprehensive sendmail error messages.
* Uses Exceptions instead of PEAR_Errors.
