What's the deal with SPF lookups?

The SPF record can include other domains or IP addresses using mechanisms like include, a, mx, or redirect. When these mechanisms are used, the receiving server must "look up" those DNS records for those domains to verify their mail servers.

For example, include:_spf.google.com means the receiving server has to check Google's SPF record too. And that record could include other lookups as well. And those records could include...you get the idea.

If the receiving email server needs to perform more than 10 lookups, it will fail the SPF check with a "PermError" (permanent error) and your email might be rejected. So, you should try to keep the number of potential lookups under 10.