Glossary · Authentication
An SPF include pulls another domain's SPF record into yours, the usual way to authorise a provider like Google Workspace or Mailchimp to send as you.
include:_spf.google.com tells the receiver to evaluate Google's SPF record as part of yours: if the sending server matches it, your record matches too. Providers publish these records so they can change their IP ranges without you editing anything.
Each include costs one DNS lookup, plus whatever lookups the included record uses, all counted against the 10-lookup limit. An include that points at a domain with no SPF record is a permanent error. Remove includes for services you've stopped using; they waste lookups and keep authorising the old provider.
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