Send email from separate domains

["banderson" <user=1590@nabble.blu.org>]

 I have an open source application (alfresco) that is using smtp to send mail to/from different users.  The problem is that it can't send mail from a user unless that user's email address has the same domain as my server.  For example, I have to use an email address like user@mydomain.com to send, but can't use user@gmail.com.  I check my /var/spool/mail/root file and it has this error: 

----- The following addresses had permanent fatal errors ----- 
<banderon1@gmail.com>
    (reason: 550 Host unknown) 

Thank you for your help.