If your SPF record causes receiving mail servers to lookup too many domains, some receiving mail servers will reject your email, even when the email itself passes all SPF/DKIM/DMARC checks.
The tricky part of that to diagnose - which [1] talks about, and links to a tool to diagnose it [2] - is that there may be additional lookups that the servers you list in your SPF cause to happen.
So you could have an SPF record with only 4 servers, but if one of those servers causes 7 additional lookups, you might have over 10 SPF lookups. 10 seems to be a growing-in-popularity limit on SPF lookups.
So even if you have SPF, DKIM, and DMARC setup, make sure you don't have too many lookups caused by your SPF record!
1. https://easydmarc.com/blog/spf-too-many-dns-lookups-error 2. https://easydmarc.com/tools/spf-lookup
