SPF Record Checker
SPF authorizes which servers may send mail for your domain. It is easy to break: a second SPF record, more than ten DNS lookups, or a too-permissive all mechanism all cause silent failures.
How it works
We fetch the domain's TXT records, find the one starting with v=spf1, and check it: exactly one SPF record (a second is a PermError), the count of DNS-lookup mechanisms against the hard limit of 10, and the all qualifier (~all softfail is the safe default; +all is dangerous).
Authentication is step one.
Once your records check out, inbox placement still depends on reputation and warmup. Scheduler Zero handles both. Need to fix a record first? See the per-host setup guides.