John and Dave,
Thank you for the good information! Your replies really saved my time for
On Fri, Apr 20, 2012 at 10:54 PM, Dave Crocker <dhc(_at_)dcrocker(_dot_)net>
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.
Like the MX record, it took some iterations to figure out the technical
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.