Wrap a few long lines.
Structure bounce content as a multi-part MIME message. Content-Description
and Status code/description is a work in progress.
550 means 'Invalid Recipient'.
Content-Description isn't mandatory.
Remote-MTA isn't the same as Reporting-MTA. Removing it until we
properly gather Remote-MTA.
Make boundary uint64_t and use generate_uid() as suggested by gilles.
Merge pull request #468 from snimmagadda/432n

Bounce Content rework #432