[mms] Added Horde_Smtp property indicating whether error is permanent or transient.
Add general category error codes if we don't specifically handle an error type
Thrown runtime exception here, since this is a coding issue
Fix encoding charset headers when no default header or body charset has been set
Ignore empty content parameters
Remove superfluous linespaces in MDN messages
Add unit test for Horde_Mime_Mdn#generate()