Validate your SPF record, count DNS lookups, and check your all mechanism.
SPF is a TXT record listing the servers allowed to send mail for your domain. Receivers check it to decide whether a sending IP is authorized.
SPF allows at most 10 DNS lookups (include, a, mx, ptr, exists, redirect). Go over and SPF returns permerror and effectively fails. This checker counts your lookups so you can flatten includes before you hit the limit.
End your record with -all (hard fail) or ~all (soft fail). +all authorizes any server and is dangerous. This tool flags it.
DeliverSignal runs these checks daily and alerts you when something breaks.
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