My esteemed colleague Brent Emerson wrote this very thorough yet readable primer on DNS and SMTP for our friends in the Tech Underground, but then decided it should just be disseminated widely, so I'm finally getting around to posting it. If you ever have to deal with DNS issues or mail delivery issues, or if you just want to plug some holes in your general understanding of how these systems work, have a look. It's written for technology integrators, but probably useful for anybody who ever has to touch DNS or email configuration for a client, including many Web developers.