John and Dave, Thank you for the good information! Your replies really saved my time for googling.
joe On Fri, Apr 20, 2012 at 10:54 PM, Dave Crocker <[email protected]> wrote: > > > On 4/20/2012 9:51 PM, John Levine wrote: > >> The longer answer is that thirty years ago, in RFC 821 there was a >> TURN command which does what you suggest, switches the roles of the >> two ends of the SMTP session. But that turns out to be a giant >> security hole, since a bad guy A' could steal mail by connecting to B >> while pretending to be A, doing a TURN, and collecting mail intended >> for A. So SMTP servers don't do that any more. >> > > check out ETRN. > > > http://en.wikipedia.org/wiki/**Extended_SMTP#ETRN<http://en.wikipedia.org/wiki/Extended_SMTP#ETRN> > > Like the MX record, it took some iterations to figure out the technical > details. > > The issue isn't slow lines but occasional connections that the 'other' > side can't predict. That is, it is for those situations in which only one > side can initiate link-level connections. > > d/ > > -- > Dave Crocker > Brandenburg InternetWorking > bbiw.net >
