On 20/04/2012 22:32, Joe Hwang wrote: > 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? What is the command or > process like? This is defined with the smtp "TURN" command. Generally considered to be a Really Bad Thing. Not even sure if it's ever been implemented anywhere. Nick