How SPF Records Work and How to Build One Safely
Understand SPF syntax, includes, DNS lookup limits, all-mechanisms, and why a domain should normally publish one SPF policy.
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The Probably Useful Guides explain the calculations, formats, web tasks, and everyday decisions behind our tools. They are written to help you understand why a result works—not just copy an answer—using practical examples, formulas, common mistakes, and limitations where they matter.
Browse the latest guides below, then jump to the related calculator, generator, or converter when you are ready to apply what you learned. Topics currently include business pricing, percentages, Canadian sales tax, campaign tracking, email authentication, redirects, URLs, JSON, encoding, passwords, and other practical web tasks.
How to Make a Clear, Professional Invoice
A practical invoice checklist covering identity, invoice numbers, line items, taxes, due dates, payment instructions, and record keeping.
Read guideHow to Calculate Percentages: 3 Useful Formulas
Learn the three percentage formulas that cover most everyday questions, with examples for discounts, ratios, increases, and decreases.
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