Kickstarter next Monday so RYOMS has to be at copyedit before then, and my wife broke her leg last week. I’m glad an episode exists, at all.
Email uses several protocols, but only one will routinely give you fits. You control both ends of a Local Mail Transport Protocol (LMTP) connection. You can set up oddball clients to duplicate a user’s IMAP configuration. DNS, TLS, these are well-understood headaches. But the Simple Mail Transfer Protocol (SMTP) underlies all of email, and you can’t possibly build test systems that replicate every whackadoo environment you communicate with. The protocol’s simplicity is a huge part of why it’s so successful, and why it’s so abused. To run your own email system you must understand SMTP’s weaknesses all the way down to your marrow. We’ll start by using the protocol, proceed to abusing the protocol, and discuss status messages, greylisting, block lists, and forwarding.
The Kickstarter’s at https://www.kickstarter.com/projects/mwlucas/run-your-own-mail-server, by the way. Tell you friends. Back early, back often.