Send on behalf schemes are the main target audience here. Given the
concentration of email recipients on over-the-top providers and the larger
ISPs, being able to unauthenticate and reauthenticate without having to get a
new connection to some very busy systems going can result in significant
savings in latency.
Oh, OK. Why wouldn't you use the AUTH= parameter on MAIL FROM?
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
"I dropped the toothpaste", said Tom, crestfallenly.