Failed to send mail to external domain via portal

Hi Gurus,

By following the configuraion instructions in SAP library - Collaboration - Groupware - Installing and Configuring E-Mail Connectivity, I managed to send mail to recipients who reside in same domain e.g. abc@mycompany.com via portal.

However, I failed to send mail to external domain e.g. abc@yahoo.com. I got the following error message:

The mail could not be sent to the specified recipients com.sap.ip.collaboration.gw.impl.transport.javamail.exception.MailSendException: The mail could not be sent to the specified recipients
	at com.sap.ip.collaboration.gw.impl.transport.javamail.JavaMailTransport.sendMail(JavaMailTransport.java:183)
....
--------- exception is chained. Original exception ------------
javax.mail.SendFailedException: Invalid Addresses;
  nested exception is: 
	javax.mail.SendFailedException: 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
	at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:804)
	at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:320)
...

Please help.

Thanks alot.