Sender Policy Framework, or SPF. SPF is an open standard that specifies a method for preventing sender address forgery, according to openspf.org. It isn’t about stopping spam; it’s about controlling and stopping attempted sender forgeries.

Kitterman.com - E-Mail SPF Testing tool
    These tools are meant to help you deploy SPF records for your domain. They use an actual RFC 7208 compliant library (pyspf) for tests and will dynamically test for processing limit errors (no other testers I'm aware of do this).
Openspf.org - In Depth SPF Testing
    OpenSPF.org provides an e-mail based record tester.
MXToolbox.com - DNS SPF Record check
    Online tool that checks your DNS SPF Records

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