What is the SPF 10-lookup limit?

An SPF record may trigger at most ten DNS lookups while it is being evaluated. Exceed that and the check fails permanently:

• Each include costs a lookup - and each included record can trigger more of its own.
• It accumulates quietly - adding a fifth or sixth service is what usually tips a working record over.
• The failure is not partial - the whole record errors out, so all your mail is affected.

Check your record against the limit whenever you add a sending service; a free SPF checker reports the count.