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:

• https://www.digitalocean.com/community/tutorials/how-to-use-an-spf-record-to-prevent-spoofing-improve-e-mail-reliability

And for checking SPF records you can use the following sites:

• https://www.kitterman.com/spf/validate.html
• https://mxtoolbox.com/spf.aspx

If you know any other site with valuable SPF information, please leave them as a comment.