The original SMTP specification described a "TURN" command that allows the roles of server and client
to be reversed in a session. When a client issues the "TURN" command, the server "turns around" and sends
any queued mail for that domain to the client, essentially treating the client as an SMTP server.
The "TURN" command is obsolete and insecure. It specifies no authentication mechanism, allowing a single
user from a domain to retrieve all queued mail for that domain (for all users). Modern SMTP servers reject
the "TURN" command for these reasons. A replacement for "TURN" command, called "ETRN", has been
proposed to rectify some of the security problems with "TURN".
However, this proposal is not without its own security problems.