From the sendmail cf/README:
nullclient This is a special case -- it creates a configuration file containing nothing but support for forwarding all mail to a central hub via a local SMTP-based network. The argument is the name of that hub. The only other feature that should be used in conjunction with this one is FEATURE(`nocanonify'). No mailers should be defined. No aliasing or forwarding is done.
However, you do not have to run sendmail1 on a server unless it is your SMTP server. Sendmail has FEATURE(`nullclient’) for that. Qmail has something similar. But there is nothing simplest than this alternative:
This is nullmailer, a sendmail/qmail/etc replacement MTA for hosts which relay to a fixed set of smart relays. It is designed to be simple to configure, secure, and easily extendable.
Nullmailer is very easy to configure compared to any of the well known F/OSS SMTP servers. It is not an SMTP server. It is a /usr/lib/sendmail2 replacement for your servers or unix shell machines.
Installing nullmailer on a Debian system simply means:
# apt-get install nullmailer
# dpkg-reconfigure nullmailer
and you are all set3. If you are on another system the worst case senario requires that you compile it by hand. However, this is not a difficult task even for the inexpirienced SysAdmin, since the code is compact and easy to read and the directions on how to compile / install fairly simple to follow.