what version of Sendmail ?

If it is the default with solaris 8 - then you should find a /etc/mail/sendmail.cf file - make a copy of it, vi the original and add the mail relay name in either the DH or DR line. Then insure that the start up script in /etc/rc2.d or /etc/rc3.d is removed. If you don't want the system to accept mail don't start Sendmail. Email will still go out to the relay.

Example:
# who I send unqualified names to (null means deliver locally)
DRsmtp.womnet.com

# who gets all local email traffic ($R has precedence for unqualified names)
DHsmtp.womnet.com

FYI - if your system is running sendmail and is connected to the internet (and not protected by a firewall) there is a possibility that a spammer can use your system as a relay. This is why you don't start sendmail on boot unless the system is the mail relay.