Avoiding FEATURE(`nullclient’)

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:

Enter nullmailer:

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.

[1] Or postfix, or exim, or qmail
[2] Yes, there was a time that the sendmail binary lived in /usr/lib
[3] Usually this is followed by a dpkg –purge exim4-base exim4-config