If you have C language programming experience, look at the conf.c file. Observe how sendmail initializes its default values and how it accommodates many operating systems. Read the code in the checkcompat() function.
Look in the contrib directory. Determine whether any of the contributed software or documents are of value to you or your site.
For a historical understanding of the philosophy behind sendmail, read the document doc/usenix/usenix.ps. If you have access to the Internet, get and read the RFCs mentioned in the Preface.