Dig Howto

Instead using nslookup use dig,  in RHEL distro, dig is part of bind-utils package.
http://www.madboa.com/geek/dig/

How to SMTP Auth in Sendmail (CentOS)

By default sendmail-cf  is not installed in CentOS:

yum install sendmail-cf

Then edit /etc/mail/sendmail.mc and add this:

TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl

compile the new configuration:

make -C /etc/mail/

saslauthd must by running:

/etc/init.d/saslauthd start 

and make it available on startup:

chkconfig  saslauthd on

then restart sendmail:

/etc/init.d/sendmail restart

and it's ready to use
For a powerful mail server, you can combine this configuration with MailScanner, clamav and grey-list.