I'm going to build a lil' app (PHP/MySQL) that will receive forwarded emails and extract some data. how can I check if a forwarded email is really from the original source/email server and hasn't been tampered with? For example, third party companies, Foo, Inc and Sprockets, LLC, emails a user their daily data and then that user forwards that daily email to my app/system. I want to make sure they sent me the original email and didn't send me a re-created email with cooked data/stats.
I do not know the best from simple methods to check the connection with server than SSL, and to check the validity of the data than electronic signature of a trusted party.