John R Levine writes:
BINARYMIME was defined in 2000 to avoid that issue, and invents
a new BDAT command that uses a byte count rather than escape
sequence, so the message body can be an arbitrary sequence of
octets. Gmail, hotmail and icloud/me.com support it, Yahoo and
AOL don't, but I've never seen client software that would take
advantage of it.
Zmailer did too, but all told it seems like one of those RFCs where the
specification's reviewers outnumber its implementations.
ietf-smtp mailing list