SPF
SPF is one DNS record with one job and a surprising number of ways to fail while looking correct.
This is the short version: what it declares, the limit that breaks it, and how to fix a record that has grown past what the specification allows.
What this covers
Chapter 1 of 5
Which systems may send as you, and nothing more than that.
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Chapter 2 of 5
A hard ceiling, and what happens the moment you cross it.
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Chapter 3 of 5
What the difference between ~all and -all actually means.
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Chapter 4 of 5
Removing, narrowing, flattening, and the risks of each.
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Chapter 5 of 5
A separate budget of ten, and a separate reputation.
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Check yours
Everything above, checked on your domain in about ten seconds. Free, no account.