SPF records only allow 10 ‘lookups’ to reduce the load on the email receivers side. The following mechanisms count as lookups:
The ‘nested’ lookups also count. If you exceed this threshold, the items after the 10th lookup may (/probably will) not count as valid SPF sources.
You will need to reduce the number of lookups. This can be achieved by:
- Cleaning up your record: Sometimes there are duplicate mechanisms in the record (for instance an MX record to Google Apps and an include from Google SPF)
- Use subdomains for specific email flows. If you set up a subdomain for specific flows you get another 10 lookups for the SPF record for that subdomain.
- Check if you’re using the correct included domains. Some third-party senders change their usage of SPF every now and then. Perhaps you’re using an old setup which leads to additional included domains (perhaps an include which is redirected to a new URL)
- Use SPF macros (advanced)