Deliverability failures are rarely loud. A DKIM key rotates and stops matching, a DNS provider changes a record, a new service starts sending from your domain without being added to SPF, and for a week your email lands in spam before anyone notices. Monitoring is the only defense, and it is easier to build than most teams expect.
Five checks catch the vast majority of real failures. SPF: fetch the record, validate the syntax, count the DNS lookups, and confirm the sending services are listed. DKIM: resolve the selector your provider uses and confirm the key is present and has not rotated to something your mailer does not sign with. DMARC: confirm the record exists, read the policy, and note whether subdomains are covered. MX: confirm the mail exchangers resolve and are reachable. IP reputation: check the sending IP against the major blocklists. A TLS check on the receiving side is a nice addition but changes less often.
Daily is the right cadence for most domains. DNS changes propagate in minutes, blocklist entries can appear overnight, and a daily check means the worst case is a day of email in the spam folder. Hourly is overkill unless you are sending at scale and a single bad hour costs real money. If you build this yourself, poll once a day at a fixed time and store the result of each check so you have a history to diff against.
Alert on state changes, not on failures. A domain that has had a broken SPF for a month is not a new problem, and paging someone about it every hour is noise. The useful alert is the flip: SPF was passing yesterday and fails today, DKIM was resolving and now returns NXDOMAIN, the domain appeared on a blocklist. That flip is the moment a human should look. Include the before and after in the alert so the fix is obvious without opening a DNS console.
Most flips are DNS. The fix is to add or correct a record, which means the monitoring system should be paired with whoever controls the zone. For a SaaS product, the natural move is to own the monitoring and send the alert to the customer, which is exactly the loop Inboxproof Pro runs: daily checks on the customer's domain, an email the moment a check changes, and a report link showing the current state. If you are building this for your own stack, a cron job, five DNS lookups, a diff against yesterday, and an email on change is a working system in an afternoon.
The fastest way to see where you stand is to run the checks now.
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