Cant send mail from server using postie

When a mail is sent from a progress database on our web server, Postie is being used to send the mail on to the relevant parties. It is only however sending mails to people internally and not externally. When looking at the x.bat file that is created each time postie sends a mail it appears that the host server is the same for people both internal and external. Postie was introduced a couple of months before I started and I’m not sure how to go about configuring it.