Zend_Mail does not check for the correct character set of the mail parts. When
Zend_Mail, a charset for the e-mail itself may be given. It defaults to
iso-8859-1. The application has to make sure that all parts added to that mail object
have their content encoded in the correct character set. When creating a new mail part, a different
charset can be given for each part.
Note: Only in text format
Character sets are only applicable for message parts in text format.