You probably need to configure the Mac OSX Server for sending e-mail.
The local server is located in the Server application under "Mail" in the left sidebar.
I recently changed to using Amazon SES and had to edit the Postfix SASL configuration manually. Edit SASL password file, run postmap
, enable SASL/TLS in main.cf
configuration file and finally restart Postfix.
This detailed description by Rojas helped me:
http://benjaminrojas.net/configuring-postfix-to-send-mail-from-mac-os-x-mountain-lion/