Malicious attachments and links remain the main payload delivery method for ransomware and credential theft.
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#
- Block executable and macro-enabled attachment types at the gateway.
- Enable URL rewriting with time-of-click reputation checks.
- Sandbox unknown attachments before delivery.
- For outbound, prefer links to authenticated portals over attachments for sensitive files.
Frequently asked questions#
Are PDF attachments safe?
Mostly, but PDFs can carry scripts and phishing links. Sandboxing and link rewriting still apply.