Originally published on the Merlonix blog.
There are two different questions about your domain's email authentication, and almost every checker answers only the first. The first is do you have SPF, DKIM, and DMARC records — a presence question, a yes/no lookup. The second is do those records actually stop someone from sending email that looks like it came from you — an enforcement question. You can pass the first and fail the second completely, and the gap between them is the whole game: a domain with all three records published, every free checker showing green, that a spammer can still spoof at will because each record is published in its permissive, do-nothing mode.
The permissive modes exist for a good reason — they're how you roll these records out without bouncing your own legitimate mail. The problem is that "published it in monitor mode so I could watch first" and "finished" look identical to a tool that only checks presence, and an enormous number of domains stop at the first and never come back. Here's what actually decides enforcement, record by record, and how to tell which mode yours is in.
SPF: only -all actually rejects
An SPF record lists which servers are allowed to send mail as your domain, and it ends in an all mechanism that says what a receiver should do with a server that isn't on the list. That final qualifier is the entire enforcement decision, and there are four of them:
