To run the sendmail redirector (SMTPRedirect.pl), copy the Perl script into /usr/sbin/SMTPRedirect (renamed without the ".pl" extension). Change the ownership and permissions so that it looks like:
-rwxr-xr-x 1 root root 53912 Nov 16 2009 /usr/sbin/SMTPRedirect
You may want to hack the default mail host ($MAILHOST_DEF) in this file to set it to your default mail host's name. Other than this, nothing is required to be done to sendmail startup, etc. to run the redirector. It is done so, automatically, whenever mail is to be sent.