I used to use the openspf.org website as a valuable resource for every SPF question I had, especially around creating SPF records. For some reason, most like funding related the openspf.org website disappeared early 2019.
Another valuable resource with information regarding SPF records is:
And for checking SPF records you can use the following sites:
If you know any other site with valuable SPF information, please leave them as a comment.
Jaap Wesselius 
Hello Jaap,
I frequently use emailstuff.org for checking SPF records (number IP’s and number of lookup’s), But for Syntax checking only reliable site is Kitterman
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I didn’t know the emailstuff.org site, but it looks very useful, thanks!
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The site is reachable here … http://www.open-spf.org/ . There’s a hyphen ‘-‘ in the middle that distinguishes it from the previous domain.
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Thanks for the feedback!
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