AllowEHLO is a Boolean property that indicates if the SMTP server implements the Extended HELLO (EHLO) command verb used to identify the SMTP client to the SMTP server. When AllowEHLO is True, it is implied that the SMTP server implements one or more of the SMTP service extensions. 

AllowEHLO is used in the TIdSMTPServer.CommandEHLO command event handler when the SMTP client has issued the SMTP EHLO command. 

When AllowEHLO contains True, the server will send the values in the greeting message assigned to the Messages.Greeting.EHLOReply property that indicate the server capabilities. When AllowEHLO contains False, the server will send the greeting message value in the Messages.Greeting.EHLONotSupported property to new SMTP client connections.

TIdCommand TIdCommandHandlers Messages TIdSMTPMessages