indy thunderbird receipt problem

hi,
i've been developing on a e-mail sender program using the indy 9
component on delphi. It doesn't have any problem in working correctly
under normal circumstances. When i request a receipt recipient i can
recieve this info through outlook however this causes a crash on
thunderbird and because thunderbird doesn^t give any kind of failure
message, i couldn't find the source of the problem.
i wonder if you have any information about this subject. Here is my
code blog i've been using. Thanks for all your help.

     mailmessage:=tidmessage;
     mail:=TIdSMTP;

     MailMessage.charset := 'ISO-8859-9';
     MailMessage.ContentTransferEncoding:= 'multipart/related';
     MailMessage.From.Address:=sSenderMail;
     MailMessage.From.Name:=sSenderName;
     MailMessage.ContentType := 'text/html';
     MailMessage.Recipients.EMailAddresses:=sToAddress
     MailMessage.Subject:=sSubject;
     MailMessage.body.Text:=sBody;

    MailMessage.ReceiptRecipient.Address:=sSender;

   Mail.Host:=sServer;
   Mail.Username:=sUserName;
   Mail.Password:=sUserPas;
   Mail.AuthenticationType:=atLogin;
   Mail.Connect;
   Mail.Send(Mesaj);
   mail.Disconnect;