Use an API to check domain reputation before allowlisting

An allowlist can stop a familiar customer, supplier or system from being treated like a stranger. It can also become an attacker’s shortcut when a domain is added too casually. An API-backed reputation check makes the decision repeatable, evidence-based and suitable for automated workflows.

If your team needs to know how to use the API to check domain reputation before adding it to your product’s allowlist, begin with the identity that appears in the message headers. The visible From domain, Return-Path domain and DKIM signing domain are related, but they are not interchangeable.

Authentication signals establish whether infrastructure is authorised to send for a domain. Reputation signals help assess whether that infrastructure has a trustworthy history. Using both provides a stronger basis for approval than relying on a domain name or logo alone.

A dependable implementation verifies, classifies, records and re-checks each decision. The API should supply evidence for a policy, while ambiguous cases remain available for manual review. Reputation changes over time, so approval on launch day is only the beginning.

Define the domain scope

Decide whether the product is allowing inbound messages, approving an outbound service provider, or handling both. Record the exact approved domain and whether subdomains are covered. A wildcard rule for *.partner.com.au should not silently approve unrelated tenants on the same platform.

Normalise names before sending them to the API: lowercase them, remove accidental protocols or paths, convert internationalised labels consistently and reject malformed input. Request reputation and authentication results for that domain, keeping the API credential on a server where browsers cannot expose it.

Treat the returned score as a point-in-time signal rather than a permanent certificate. Combine it with domain age, sending identity, authentication alignment and any risk reasons returned by the API. The platform’s guidance on regular trust-score reviews explains why trusted sending domains should be reassessed after configuration, traffic or ownership changes.

Build the verification workflow

Map the API response to a clear internal policy such as approved, restricted, blocked or requires review. A high score with valid DMARC and DKIM may qualify, while a low score, failed alignment or conflicting identities should not. Define in advance how missing data, rate limits and service timeouts are handled.

Keep checks idempotent so repeated submissions cannot create conflicting entries. Cache results briefly by exact domain and policy version, then refresh them before a material transaction such as changing bank details, restoring access or enabling an integration. Record the domain, timestamp, score, reasons and decision, while avoiding unnecessary duplication of personal information.

When a vendor sends on your behalf, verify both the organisation and the domains it signs. Require a sample message and compare its visible From domain with the authenticated Return-Path and DKIM domains. This prevents a reputable domain from being approved merely because it appears in a forwarded email header.

Apply risk-based rules

Choose thresholds that reflect the consequence of each message type. A signup confirmation can tolerate a review state, while payment, identity and account-recovery messages require stricter controls. Passing an SPF test alone is insufficient because SPF identifies authorised senders without proving that the underlying business is legitimate.

Read DMARC results in context. An aggregate report showing many rejections may reflect deliberate quarantine of spoofed mail rather than a broken domain. This high DMARC rejection rates guide explains how to distinguish protective rejection from an authentication problem before deciding whether a sender belongs in the allowlist.

Document every exception. A known Australian supplier may still be quarantined if a new sending subdomain lacks proper alignment. Record who approved the exception, when it expires and which compensating controls apply. Avoid permanent “trusted” bypasses, since compromised credentials can turn a temporary exception into a persistent route for abuse.

Fit Australian operating requirements

Where the Privacy Act 1988 applies, logs containing customer names, email addresses or IP addresses can engage the Australian Privacy Principles. Apply data minimisation, restrict access, set retention periods and secure the audit trail. Keep trust decisions separate from marketing data, and do not disclose customer domains to an external service unless your privacy notice and governance arrangements support that use.

Australian Communications and Security Centre advice on phishing and Scamwatch guidance on verifying unusual requests remain relevant after technical approval. Staff should independently confirm urgent changes to bank details or credentials through a known channel, especially if a supplier requests a payment-account switch by email.

Many Australian teams use Google Workspace, Microsoft 365 and mobile Outlook, while .au and .com.au business domains are commonplace. These conventions aid recognition but create no automatic trust advantage. In regulated or high-risk settings, such as fintech operations in Sydney, Melbourne and Brisbane, allowlist decisions should also consider authentication evidence, complaint history and the proposed business purpose.

Monitor and revoke access

Schedule API re-checks for every approved domain, with closer monitoring for finance, login recovery and high-volume notifications. Trigger an immediate review when DKIM keys rotate, DMARC policies change, a new sending subdomain appears or support reports increased spoofing. Version the policy so reviewers can reconstruct why a domain was admitted.

Have a response ready if a previously trusted domain suddenly scores poorly. The guide on respond to score drops covers possible causes such as changed authentication, reputation events or suspicious traffic. Restrict a questionable domain while its owner and security team investigate, rather than removing a critical provider without assessing operational impact.

Keep outcomes visible in the product and operational records. When access is removed, update related allowlists and caches and notify the owner through a previously verified channel. Restoration requires fresh API and authentication results instead of reinstating the previous status automatically, creating a repeatable control with a clear audit history.