Thank you!

All of my mail passes through Exim. Though the code is, well, interesting at places, as a system administrator it's generally great stuff.

I see more and more email coming in with invalid envelope senders, though, and callouts tend to trigger alerts in spam filters. So I'm not sure if this is as universal as the quote would have it to—certainly, if e.g. Gmail did such checks, nobody would send from junk addresses.