We already have a large, distributed, heterogeneous, deployed
email architecture infrastructure and MTAs. Extensions need
to work with that existing, deployed infrastructure in a
backwards-compatible way. Believe it or not, the world isn't
going to switch over to completely new MTAs overnight just
because some draft says so.
Indeed we took great care of the incremental deployment issue because we
understand the value of the infratrstucture.
1) It is easy to see that if a receiver does not support DMTP, then
everything will be fine (it does not matter the sender supports DMTP or
2) Assuming the receiver supports DMTP, then we have two cases:
2.1) the sender supports DMTP. If so, the receiver will know this because
of the extension we added in the ELHO command (admitted: we may have not
followed the standarded way to extend, we will make changes in the new
draft). So the sender and the receiver can proceed according to DMTP.
2.2) the sender does not support DMTP, the receiver will also know this
from the extension, so they proceed using the incremental deployment