How to refuse sendmail in SMTPServer?

Everybody!
  In my SmtpServer,I check user,if user is local,then i allow user sendmail,
if not,I refuse,but,In procedure SMTPServerCommandMAIL,I write
---------------------------
Procedure TfrmMain.SMTPServerCommandMAIL(Const ASender: TIdCommand;
  Var Accept: Boolean; EMailAddress: String);
begin
    Accept := IsLocalUser(EMailAddress,ASender.Thread.ThreadID );
End;
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Even Accept=False,but Client can send mail.
How to do? and why?

Thanks!