On Fri, Feb 02, 2007, Eric A. Hall wrote:
ahh wait, linear responses are easier for the client but introduce more
work to the server, since they have to keep track of status for all
recipients (even the nonexistent ones) across the entire session.
The server needs to keep only track of RCPTs that it accepted:
4.2. The DATA command
...
The change is that after the final ".", the server returns one reply
for each previously successful RCPT command in the mail transaction,
^^^^^^^^^^^^^^^^^^^^^^^^^^