Automating email blocking with sender trust scores

A domain trust score can help security teams decide whether an incoming message deserves further inspection, quarantine or rejection. By connecting the Trusted Sender Score API to a mail gateway, organisations can turn reputation data into an automated control instead of relying on manual checks for every suspicious sender.

The safest design treats the score as one signal within a broader email security policy. SPF, DKIM, DMARC alignment, sender history, threat intelligence and business context should influence the final decision. A low score can trigger blocking, while an uncertain result can send the message to quarantine for review.

Define what the score means

Before setting a threshold, establish what the score measures and how frequently it may change. A domain with a low reputation could be associated with spoofing, weak authentication, malicious activity or incomplete ownership information. It does not automatically prove that every message from that domain is harmful.

Choose separate actions for different score ranges. For example, a high score may allow normal delivery, a middle range may require additional authentication checks, and a score below the selected threshold may result in quarantine or rejection. This graduated model reduces the chance of blocking a legitimate supplier because of a temporary reputation issue.

Prepare domains and authentication data

Start by compiling the domains your organisation receives mail from regularly. Include suppliers, payment providers, software platforms, recruitment services and marketing systems. Review their SPF, DKIM and DMARC configuration, since a trustworthy domain with poor authentication still deserves careful handling. The DMARC project guide provides useful background for connecting reputation checks with domain authentication.

For internal domains, verify ownership and administrative access before automating policy changes. This is particularly important for businesses using several .au domains, subsidiaries or outsourced mail services. The process for becoming a domain admin can help ensure that the people managing scores and authentication settings have appropriate authority.

Create the scoring decision

Your integration should send a domain or sender identity to the API, validate the response, and compare the returned trust score with a configured threshold. Keep the threshold in application settings rather than hard-coding it, so your security team can adjust the policy without redeploying the entire mail system.

A simple decision model might look like this:

if API result is unavailable:
    quarantine for review
else if score < threshold:
    reject or quarantine
else:
    continue with authentication checks

Avoid treating an API timeout as a low score. A service outage and a genuinely poor reputation are different events, and confusing them can create unnecessary disruption.

Connect the API to your mail gateway

The API call can run when the receiving server first sees the sender domain, during message inspection, or through a security orchestration platform. Cache results for a suitable period to reduce repeated requests, improve response time and stay within any usage limits. Refresh cached values regularly so an old positive result does not remain trusted indefinitely.

Use secure API credentials stored in a secrets manager, rather than placing keys in mail-server configuration files or source code. Apply rate limits, request timeouts and structured error handling. Where the mail gateway supports it, use quarantine as the first automated response while the integration is being tested.

Handle Australian business traffic carefully

Australian organisations often receive mail across multiple time zones, particularly when operations span Sydney, Melbourne, Brisbane, Perth and overseas offices. A short outage or an overly strict rule can delay invoices, payroll notices and customer messages outside local support hours. A quarantine queue with clear escalation ownership gives staff a way to recover legitimate mail quickly.

Consider sector-specific sensitivity as well. A healthcare provider, school, council or NDIS service may receive messages from many external domains, while a small professional firm may have a much narrower sender profile. For Australian businesses subject to privacy obligations, limit stored message data and retain only the domain, score, decision, timestamp and technical identifiers needed for investigation.

Log, review and tune the policy

Record each API response and enforcement action in a central security log. Useful fields include the sender domain, score, authentication results, threshold version, message identifier and reason for the decision. These records help identify false positives and show how an automated control operated during an incident.

Review blocked and quarantined messages with the people responsible for finance, operations and customer service. A legitimate relay may appear suspicious when its visible From address differs from the infrastructure delivering the message. Guidance on legitimate relays can help analysts distinguish authorised delivery services from spoofing attempts.

Protect the workflow from abuse

Attackers may try to exploit an automated blocking system by causing excessive API requests, manipulating sender identities or targeting a trusted domain after its reputation changes. Protect the integration with input validation, request quotas and monitoring for unusual lookup volumes. Strip irrelevant message data before submitting a domain for analysis.

Maintain an allowlist only for carefully verified business-critical services, and review it regularly. An allowlist should never bypass every security check; messages should still pass SPF, DKIM, DMARC and malware inspection where available. Test policy changes with representative Australian sender traffic before enforcing them across the whole organisation, then adjust the threshold using measured delivery results rather than guesswork.