I see insufficient value for this extension between mail domains or across
the general Internet to justify the risk to the infrastructure. However,
this extension appears well suited for mail submission. It provides a way
for a ligher-weight client to submit a raft of message to a submission
server that can then assume value-added post-processing responsiblities.
The extension off-loads an enormous amount of traffic to a presumably
better-connected, specialized mail server from a less well connected client.
As such, the extension would be useful to standardize even if explicitly not
recommended for inter-domain use.
Use with submission would also address many of the CPU and SPAM concerns
raised by others by making the use of the extension subject to the policy of
the submission server owner and limiting the extra processing burden to that
server.
Greg V.