CheckUser is a TUserHandler event handler property that represents the event handler signalled when a TIdCommandHandler is used to perform authentication for a threaded SMTP client connection. 

CheckUser receives arguments that include the TIdCommand being executed, the Username and Password from the client thread, and an Accepted flag to indicate if the user authentication is valid. 

Applications must assign a propcedure to the event handler to allow responding to the event notification, and to implement the processing required to perform authorization of the client account. 

Set the value of the Accepted argument to True to indicate successful authentication, or False when the Username or Password cannot be authenticated. 

When CheckUser is not assigned, the account authentication is always accepted.

TIdCommand TIdCommandHandler TIdCommandHandlers TUserHandler