>Let's say that there are two SMTP servers, A and B, and A initiates a >session (TCP connect to B). After A finished one transaction (transferring >some email content from A to B), is it possible B starts transferring some >email content to A using the same TCP connect? The short answer is no. 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. Keep in mind that the number of TCP packets needed to send a TURN command and then receive the 250 confirmation message is no less than the number needed to set up a new connection, so even if it were possible to fix the security problems, TURN would have little practical benefit. RFC 821 says it was intended for connections like telephone dialup where setting up a new connection really is slow. R's, John