Definition
SPF is a TXT record published at the root of the domain. It starts with v=spf1 and lists the authorised sending servers: IP addresses, or include mechanisms pointing to a provider's list (mailbox, email marketing tool, CRM). It ends with -all, no other server is authorised, or ~all, another server is probably not authorised.
The standard (RFC 7208) allows a single SPF record per domain and at most ten DNS lookups during the check: include, a, mx, ptr, exists and redirect count, nested includes too. Beyond that, SPF fails for every message.
Value for a business owner
Gmail asks every sender for SPF or DKIM, and both from around 5,000 messages a day to its personal accounts; Outlook.com requires it to pass above 5,000 emails a day. A missing or broken SPF sends quotes and invoices to the spam folder.
At Stratedge Consulting
We list the tools that send on behalf of your domain and write an SPF record that covers them within ten lookups. Our free SPF, DKIM and DMARC test counts those lookups for your domain.
Common mistakes
Adding a second v=spf1 record for a new tool instead of extending the first: SPF becomes invalid.
Stacking includes until the check needs more than ten DNS lookups: the record looks complete, but the check fails.