Outlook, Outlook Express and Windows Mail require LOGIN SMTP authentication mechanism. This is disabled by default.
Edit /etc/postfix/main.cf and change the line
smtpd_sasl_security_options = noanonymous, noplaintext
into
smtpd_sasl_security_options = noanonymous
Now run <cli> # /etc/init.d/postfix reload </cli> to reload postfix.
Test this with
<cli>
# telnet your-server 25
</cli>
and type ehlo client
Postfix should respond with
... 250-AUTH NTLM PLAIN CRAM-MD5 LOGIN DIGEST-MD5 250-AUTH=NTLM PLAIN CRAM-MD5 LOGIN DIGEST-MD5 ...
The AUTH-lines should list PLAIN and LOGIN and Outlook SMTP authentication should work fine now.