OnCommandAUTH is a TBasicHandler event handler property that represents the command event handler used to implement the SMTP AUTH command verb. OnCommandAUTH allows the server to perform custom authentication methods or protocols not found in the default server implementation in CommandHandlers. 

OnCommandAUTH is used in the AUTH Command handler when the client connection has provided identication using the SMTP EHLO command. 

Applications must assign a procedure to the OnCommandAUTH event handler to allow responding to the event notification. When OnCommandAUTH is not assigned, the server uses the internal authentication mechanism, equivalent to AUTH LOGIN, and the CheckUser event handler. 

When the client connection uses the HELO command, authentication is not performed according to the SMTP protocol.