Try the command
hostname -f to see what your fully qualified name is. If you don't have a static address with the appropriate DNS entries use your ISP's relay to send email.
If you are sending email as localhost or localhost.localdomain, it will run into problems with spam filters. Your mail server must use its FQDN (fully qualified domain name) to send to the Internet (not required for your ISP's relay server).
Try sending email to an external email account you have access to and check the headers there. It is common to see email sent to the mail server from localhost or localhost.localdomain. The mail server must then use its name to forward the email.
Normally, your email server should known its FQDN so you should get received lines like:
Received: from localhost.localdomain ([127.0.0.1])
by mail.example.com (qmail-ldap-1.03) with SMTP
for <email@example.com>; 29 Nov 2010 00:11:37 -0000
Received: from mail.example.com ([192.0.2.15])
by mx.example.net with ESMTP
for <firstname.lastname@example.org>; 29 Nov 2010 00:11:39 -0000