Exim documentation seems to allow to delimitate better the parts (See "22. String testing conditions") :
Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
although I'm not sure regarding the variable $h_from, and I can't do too much experiments in my server. I have some hysterical customers who behave like drug addicts when a message has an special delay, not mention when a message is lost.