Once your simple three-line null.mc file is ready, you can use it to create a sendmail configuration file. You need a modern version of m4(1) to do this. If you are running BSD 4.2 UNIX, or SysV.2 or 7th edition UNIX, you'll need either the GNU version of m4(1) or BSD's Net/2 version. On SunOS systems, you can run /usr/5bin/m4.
Once you have a good version of m4(1), run the following simple command:
m4 null.mc > client.cf
If your version of m4(1) is too old, you will see this error message:
You need a newer version of M4, at least as new as System V or GNU m4: file not found: NoSuchFile
On the other hand, if your version is new enough, you will see nothing but your prompt return. If so, you have succeeded in creating a client.cf file.