SMTP really is such a pile of junk piled on top of crap piled on top of stuff piled on top of what used to be a nice simple protocol, isn't it.
Every so often, I need to chase down some aspect of email validation (#SPF, #DMKIM, #DMARC, ...). This involves a number of #DNS records and queries, but I may forget just which ones. So here's a quick #SMTP/DNS cheatsheet: