What does an SPF checker do?

It fetches the TXT records of a domain, finds the SPF record (v=spf1 ...) and validates it: exactly one record must exist, the mechanisms must stay within 10 DNS lookups, and the all-qualifier should be ~all or -all. Failures on any of these make receivers ignore SPF entirely.

Why is having two SPF records a problem?

RFC 7208 requires exactly one SPF record per domain. Two records cause a permanent error (permerror) at the receiver, which is treated as SPF failing for every email you send. Merge the mechanisms of both records into a single v=spf1 line.

What is the SPF 10-lookup limit?

Receivers stop evaluating SPF after 10 DNS lookups. Every include, a, mx, ptr, exists and redirect counts, including the ones nested inside your ESP's include. Going over returns permerror and your SPF silently stops working.

Does a valid SPF record guarantee inbox placement?

No. SPF is one of three authentication pillars alongside DKIM and DMARC, and Gmail and Yahoo require all of them for bulk senders. Run the full deliverability test to check the other two, and warm up your domain before sending volume.