Snow Leopard Server implements several anti-spam measures in its default Postfix configuration. Unfortunately there are a few glitches, which may prevent your own Outlook clients from sending mail.

Among the different parameters, there is one which explicitely verifies that the sending mail client does have a fully qualified hostname. The reason being that much spam is sent from poorly configured mail servers or compromised clients.

Outlook does unfortunately not identify itself correctly and thus gets blocked by Postfix.

To get around this, you first will need to make sure that you authenticate to send and then make a change in the Postfix configuration.

Edit

/etc/postfix/main.cf

and change:

smtpd_helo_restrictions = reject_invalid_helo_hostname reject_non_fqdn_helo_hostname

to:

smtpd_helo_restrictions = permit_sasl_authenticated permit_mynetworks reject_invalid_helo_hostname reject_non_fqdn_helo_hostname

When done, issue:

sudo postfix reload