The following is a list of additional information sources about sendmail.
Costales, Bryan. sendmail, Third Edition. O'Reilly & Associates, Inc., 2002.
Home page for sendmail – https://www.sendmail.org.
FAQ for sendmail – https://www.sendmail.org/faq.
README for new sendmail configuration files – https://www.sendmail.org/m4/readme.html.
A guide for issues that are related to migrating to more recent versions of sendmail – https://www.sendmail.org/vendor/sun/.