Noturn, turn, turn in, turn out channel options

SMTP TURN command channel options
RFC 821defined an optional TURN command, for the client (sender) and server (receiver) to switch roles. It is not normally appropriate to enable use of TURN: it is quite dangerous, as it allows any arbitrary client to "snatch" your messages! Use of the default  is thus STRONGLY RECOMMENDED!

For a safer "relay upon demand" feature, see the ATRN SMTP extension (RFC 2645).

See also:
 * SMTP and LMTP protocol channel options
 * Channel options