Building an Automated Inbound Email Trust Filter

An inbound email filter can use sender-domain reputation as one signal in a broader decision process. When a domain falls below a defined trust score, the workflow can reject the message, place it in quarantine, or route it for additional inspection before it reaches staff.

This approach is useful for Australian organisations dealing with invoice fraud, credential theft and fake delivery notifications. A domain reputation check can add context to SPF, DKIM and DMARC results, helping security teams distinguish a genuinely poor sender from a technically misconfigured but legitimate business.

The safest design treats the score as a control signal rather than an automatic verdict. A practical workflow combines reputation data with authentication, threat intelligence, allowlists and message behaviour, then records every action so analysts can review false positives and adjust the policy.

Define the trust policy

Start by deciding what each score range should mean. For example, domains below a lower threshold may be rejected at the email gateway, while those in a middle band are quarantined for review. Higher-scoring domains can continue through normal spam and malware inspection. The exact values should reflect your platform’s scoring model, risk tolerance and historical mail traffic.

Create separate rules for domains, sending IP addresses and individual messages. A low domain score is a useful warning, but it does not prove that every message from that domain is malicious. A legitimate supplier could have a poor reputation after a compromised mailbox, while a newly registered phishing domain may have little history and require additional checks.

Your policy should also account for trusted partners, government services and critical suppliers. An Australian organisation may receive genuine mail from .au domains, cloud-hosted platforms and overseas providers, so a blanket domain block can disrupt payroll, logistics or customer support. Use a narrowly scoped allowlist with an expiry date rather than permanent exceptions.

Collect and evaluate sender signals

The workflow begins when the mail gateway receives a message. Extract the visible From domain, envelope sender, return-path domain, sending IP, authentication results and any redirected or relayed identities. Then query the sender reputation service through an API or scheduled lookup. Bulk checking can help establish a baseline for common suppliers before enforcement starts.

Evaluate SPF, DKIM and DMARC alongside the reputation score. A DMARC failure with domain misalignment should increase the risk decision, particularly when the sender is impersonating a bank, council or well-known retailer. If a message passes authentication but the domain score is low, quarantine may be more appropriate than immediate rejection. Guidance on relay or spoof checks can help clarify unusual forwarding arrangements.

Keep the decision record detailed. Store the score, lookup time, authentication results, rule matched, action taken and message identifier. Reputation changes over time, so a cached result should have a short lifetime and a clear fallback behaviour if the scoring service is unavailable.

Connect the rules to your mail gateway

Most modern secure email gateways, Microsoft 365 environments and Google Workspace deployments can apply transport rules based on headers, authentication outcomes, sender domains or threat classifications. A small integration service can call the reputation API, add a trusted internal header, and let the gateway apply the final action. Avoid allowing external senders to create or modify the header used for enforcement.

A simple decision sequence is: check the allowlist, retrieve current reputation data, validate authentication, calculate the risk level, then reject, quarantine or deliver. Rejection should return a clear SMTP response without revealing internal scoring details. Quarantine should preserve the message and notify the security team rather than the recipient, reducing the chance that a malicious sender can pressure staff into releasing it.

Document ownership and data handling before enabling automation. The service terms explain the platform’s legal framework, while your organisation still needs its own retention, access-control and incident-response rules. This is especially relevant for Australian businesses handling personal information under the Privacy Act and for teams operating across Sydney, Melbourne, Brisbane or remote regional offices.

Test before enforcing automatic blocks

Run the workflow in monitor-only mode for at least several business cycles. Compare low-scoring senders with user reports, help-desk tickets and known supplier records. Pay close attention to Australian payment platforms, real-estate agencies, healthcare providers and government correspondence, where a false positive may delay a time-sensitive transaction.

Use quarantine as the first enforcement stage for borderline scores. Analysts can release valid messages, mark harmful ones as confirmed threats and identify recurring senders that need remediation. A supplier that repeatedly fails DMARC may need to correct its DNS records rather than receive a permanent exemption.

Test common edge cases: forwarded mail, mailing lists, shared cloud infrastructure, newly registered domains, internationalised domain names and compromised trusted accounts. Measure false positives, false negatives, average review time and the percentage of messages stopped before delivery. These figures provide a stronger basis for changing the threshold than an arbitrary score.

Maintain the control over time

Reputation policies need ongoing maintenance because sender behaviour, DNS configuration and attack campaigns change. Schedule regular reviews of threshold performance, expiring allowlist entries and domains that have moved between quarantine and rejection. A domain that was safe last month may now be hosting a phishing campaign, while a small business may have repaired its authentication setup.

Add an escalation path for suspected compromise. If a known partner’s score drops sharply, temporarily quarantine its messages, verify the change through an independent contact channel and notify the relationship owner. Do not rely on a reply to the suspicious email or on a phone number contained in its signature.

BIMI can provide another visible trust signal for participating senders, although it should complement rather than replace authentication and reputation checks. The review of BIMI trust signals explains how brand indicators fit into sender verification. With measured thresholds, auditable decisions and regular review, automatic blocking becomes a controlled security process rather than a brittle blacklist.