2 articles on SPF.
A record that reads perfectly can fail for every message you send. Here is how the limit works, why it is so easy to blow, and the four ways to get back under it.
Three records, three different jobs, and one specific way they combine that almost nobody explains properly: alignment. Here is the whole model in one page.
Every article here ends in the same place: find out what your own domain actually does. Check the records in a few seconds, or send one real message and get the whole picture.
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