You will probably want to replace this actual copy of sendmail with the compatability app or strange things might happen (or more likely nothing will happen). The script can be found in in the bin subdirectory of the postfix source directory if you built it from source. If you didn't, then you may need to reinstall the file some other way.
A point to note is that if you copy the file from the bin subdirectory of your original source directory, then you should perform a chmod -r sendmail command on it or you will get complaints in your mail log about security risks.

