The TURN command may be used to reverse the roles of the two programs communicating over the transmission channel.
If program-A is currently the sender-SMTP and it sends the TURN command and receives an ok reply (250) then program-A becomes the receiver-SMTP.
If program-B is currently the receiver-SMTP and it receives the TURN command and sends an ok reply (250) then program-B becomes the sender-SMTP.
To refuse to change roles the receiver sends the 502 reply.
Please note that this command is optional. It would not normally be used in situations where the transmission channel is TCP. However, when the cost of establishing the transmission channel is high, this command may be quite useful. For example, this command may be useful in supporting be mail exchange using the public switched telephone system as a transmission channel, especially if some hosts poll other hosts for mail exchanges.