phpdoc
Filtering of results should be opt-in, not opt-out

Much more intuitive
Disposition-Notification-To is an address header
Optimize/more robust address handling in Mdn object

No need to convert from mail object -> text -> mail object again.
Fix duplicate internal call to getMdnReturnAddr()
Maillog types should be externally specified by classname, not string identifier
[mms] IMP no longer uses MDN status stored in the maillog backend by default.
Improved/fix determining whether MDNs are available in a mailbox
Revert "[mms] Don't clone objects when returning data from the structure object."

This reverts commit e3e7fcdacbf34a6863474a5426d0a661e3ee952e.

This is not correct, since we want the data stored in the cache to be
pristine.

See Bug #13700
Bug #13700: Don't clone contents if they don't exist

This makes it look like the contents have always been set in the part,
even though that may not be true.