OnCommandTURN is a TBasicHandler event handler property that represents the event handler signalled when executing the SMTP TURN command verb. 

OnCommandTURN allows the server to perform role negotiation with another SMTP host, where the receiving server will begin to act as a sending server, or refuse the role negotioation. 

The SMTP TURN command, described in RFC 821, raises important security issues since, in the absence of strong authentication of the host requesting that the client and server switch roles, it can easily be used to divert mail from its correct destination. Its use is deprecated in RFC 2821. SMTP systems should not use it unless the server can authenticate the client.

TBasicHandler Messages TIdCommandHandlers TIdCommand