Zend_Mime_Message usually creates and uses its own
Zend_Mime Object to generate a boundary. If you need to define
the boundary or want to change the behaviour of the Zend_Mime
object used by Zend_Mime_Message, you can instantiate the
Zend_Mime object yourself and then register it to
Zend_Mime_Message. Usually you will not need to do this.
setMime(Zend_Mime $mime) sets a special instance of
Zend_Mime to be used by this
Zend_Mime_Message
getMime() returns the instance of
Zend_Mime that will be used to render the message when
generateMessage() is called.
generateMessage() renders the
Zend_Mime_Message content to a string.