How to bulk-check domains in SIEM logs for compromised senders
A SIEM can reveal suspicious email activity long before a complaint reaches the help desk. By extracting every sender domain from authentication failures, phishing alerts, mailbox rules and threat-intelligence events, security teams can compare domain reputation and email controls at scale.
A bulk domain review is especially useful when an organisation has several brands, subsidiaries, suppliers or cloud email tenants. For Australian businesses operating across Sydney, Melbourne, Brisbane and Perth, it can also reduce the risk of treating a regional campaign as an isolated incident.
| Approach | Useful signal | Best use | Main limitation |
|---|---|---|---|
| Manual lookup | Reputation or authentication result | Small investigations | Slow and inconsistent |
| SIEM query | Frequency, users and event timing | Finding patterns | Depends on clean log fields |
| Bulk domain check | Reputation, DKIM and DMARC posture | Large domain sets | Requires normalisation |
| API workflow | Scheduled or real-time scoring | Continuous monitoring | Needs integration and rate controls |
Prepare reliable domain data
Start by identifying which SIEM sources contain sender information. Useful fields include the RFC 5322 From address, envelope sender, Return-Path, DKIM signing domain, DMARC organisational domain and URLs found in message bodies. These values can differ, so preserving the original field helps analysts understand how a message was delivered and displayed.
Export the domains to a CSV or JSON file, then remove protocols, paths, ports, leading www prefixes and accidental punctuation. Convert names to lowercase, decode punycode where appropriate, and deduplicate them. Keep the original event ID, timestamp, recipient, sender address and source system beside each normalised domain so a reputation result can be traced back to evidence.
Give priority to domains seen in quarantine events, impossible-travel alerts, suspicious forwarding rules and failed SPF or DMARC checks. Internal domains, newly registered lookalikes and unfamiliar suppliers should remain in the review set rather than being excluded simply because they appear infrequently.
Run authentication and reputation checks at scale
Submit the cleaned list to a bulk domain checking workflow. Review domain trust, available DKIM information, DMARC policy, spoofing indicators and other reputation signals together. A domain with p=none, missing DKIM selectors or inconsistent alignment deserves a different response from one with a strong enforcement policy and stable sending history. Teams refining policy can also consult these DMARC implementation tips.
For recurring monitoring, connect the SIEM or a small internal service to an API rather than relying on analysts to upload files manually. Schedule checks after daily log ingestion, store results with a timestamp, and compare changes over time. A sudden shift from a trusted result to a high-risk result can create a useful alert, especially when it coincides with a new sender IP or unusual recipient activity.
Set practical thresholds. For example, escalate domains that combine a poor reputation with authentication failure, or domains that send to many users while appearing only once in historical logs. Avoid making a block decision from a single score; the result should guide investigation and containment.
Distinguish spoofing from an abused account
A forged sender often fails authentication or uses a lookalike domain, but a compromised mailbox may send from a legitimate domain with valid credentials. Compare the sender domain with the authenticated identity, sending infrastructure, login location, user-agent data and normal communication patterns.
A valid DKIM signature proves that an authorised system signed the message; it does not prove that the content is safe or that the sender’s account was not abused. This is why analysts should understand how a valid DKIM can mislead when reviewing campaigns that use trusted infrastructure.
In Australia, a payroll-themed message timed around the end of the financial year may appear credible even when it originates from a hijacked supplier account. Correlating sender-domain results with Microsoft 365 audit logs, mailbox forwarding changes and unusual sign-ins can separate impersonation from genuine business communication.
Correlate bulk results with SIEM evidence
Create a domain-centred investigation view containing the first-seen time, last-seen time, message volume, affected users, authentication outcomes, URLs, attachment hashes and reputation changes. This lets an analyst move from a suspicious domain to the exact messages and accounts involved without repeating searches across multiple dashboards.
Use risk tiers that match your organisation’s response process. A high-risk domain tied to credential harvesting may require message removal, URL blocking, password resets and token revocation. A domain with weak DMARC but no malicious activity may need monitoring and a notice to the domain owner instead.
Pay attention to local business relationships. A Melbourne retailer may legitimately receive mail from a logistics partner in regional New South Wales, while a newly observed lookalike using an extra hyphen warrants scrutiny. Allow-list decisions should be based on verified ownership and behaviour, not simply on a familiar company name.
Turn findings into repeatable protection
After confirming a compromised sender, preserve relevant headers and SIEM events, notify affected users, remove malicious messages and investigate account or application credentials. For an internal domain, review registrar access, DNS changes, mail-flow rules, DKIM keys and DMARC alignment. For a supplier, share evidence through an agreed security contact rather than relying on an unverified address in the suspicious message.
Add confirmed indicators to detection rules and watch for related domains, newly registered variants and reused URLs. Australian organisations can align this process with their broader ACSC-informed incident response practices and Essential Eight controls, particularly identity protection, patching and restricting administrative access.
A bulk check should become part of the detection cycle: ingest domains, normalise them, assess trust and authentication, correlate results with user and network activity, then record the decision. With clear ownership and scheduled rechecks, anti-spoofing guidance can support stronger controls across internal domains, suppliers and customer-facing brands.