OnCommandSEND is a TBasicHandler event handler property that represents the event handler signalled when executing the SMTP SEND command verb using CommandHandlers. 

OnCommandSEND allows the SMTP server to initate a mail transaction where mail data is delivered to one or more terminals. 

OnCommandSEND is used with the list of hosts and the sender email address found in the TIdCommand.Params values found in ASender. 

When the list of hosts is present, it is a reverse source route and indicates that the mail was relayed through each host on the list. The first host in the list was the most recent relay. This list is used as a source route to return non-delivery notices to the sender. 

An SMTP server implementation must assign a preocedure to the OnCommandSEND event handler to allow responding to the event noticication. When OnCommandSEND is unassigned, the text in Messages.ErrorReply is sent to the client connection.

Messages TBasicHandler TIdCommandHandlers TIdCommand