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. email@example.com via portal.
However, I failed to send mail to external domain e.g. firstname.lastname@example.org. 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) ...