Due to the irritating nature of spam email, companies have started hardening their policies for how they handle emails that originate from various sources. A recent change that has hurt a lot of sites and mailing lists lately is that email providers have begun to make their DMARC policies very strict. Specifically, email providers are saying, "If the email says that it is from one of our email servers, but it didn't come from our server, then deny the message." (I'm looking at you, Yahoo! and AOL!) This in and of itself is not the whole problem. The recipient email service must agree to respect DMARC policies. When they do, emails that say that they are from a domain, but are from somewhere else, are silently rejected.
"But wait," you say, "this sounds totally reasonable!" You're right. And wrong. And right. Let's just get this sorted out.