If you only want to send out-going mail from the MAC OS X through your linux desktop. You can install sendmail on your linux desktop. Then configure these files:
/etc/mail/relay-domains contains a
list of hosts which are allowed to
relay mail through your mail server.
This list may consist of either
specific hosts or whole domains.
/etc/mail/sendmail.cw (after sendmail
version 8.10, this file is
local-host-names) contains a list of
domains for which your mail server
will accept mail. This list is usually
the domains hosted by your machine.
You might to need to configure the OS X server to send mail to your linux box. Here is an instruction on how to do that.
This solution configures postfix, the service used by mail and sendmail, to relay messages through a third-party server (ideally your ISP), optionally using authentication and TLS. You'll need to be root to create/edit the files and run the commands. So, without further delay, enjoy.
Edit /etc/postfix/main.cf and add the
following to the end:
relayhost = smtp.yourisp.com # (you can use smtp.yourisp.com:port, such as smtp.yourisp.com:587)
smtp_sasl_auth_enable = yes
smtp_use_tls = yes
smtp_enforce_tls = yes
smtp_tls_loglevel = 2 # optional if you wan to see what's going on with the TLS negotiation in /var/log/mail.log
smtp_sasl_password_maps = hash:/etc/postfix/smtp_sasl_passwords
smtp_tls_per_site = hash:/etc/postfix/smtp_tls_sites
tls_random_source = dev:/dev/urandom