A taken-over mailbox bypasses every authentication control and becomes a launchpad for BEC and internal phishing.
The metrics that matter#
- Authentication pass rate: share of mail passing SPF, DKIM, and DMARC alignment. Healthy: 99% or higher.
- Spam complaint rate: Gmail enforces at 0.3%; stay under 0.1%.
- Hard bounce rate: under 2%; above 5% triggers platform reviews.
- Inbox placement: above 90% across major providers via seed tests.
- Engagement: click and reply rates by segment; opens are unreliable after Mail Privacy Protection.
Where to read them#
- Google Postmaster Tools for Gmail domain reputation, spam rate, and authentication.
- Microsoft SNDS and JMRP for Outlook.com IP reputation and complaints.
- Your ESP's delivery and bounce reports.
- DMARC aggregate reports for authentication by source.
Improving the numbers#
- Enforce phishing-resistant MFA (FIDO2 keys or passkeys) for admins and finance.
- Review third-party OAuth grants monthly and restrict user consent.
- Alert on impossible-travel logins and mass downloads.
- Have a documented response: revoke sessions, reset, audit rules and forwards, notify affected parties.
Frequently asked questions#
What are signs of a compromised email account?
Unknown sent items, new forwarding rules, missing mail, login alerts from unfamiliar locations, and contacts reporting odd messages.