Security Guide
How to Improve Microsoft 365 Email Security
Email is still the most common way attackers get in, and default Exchange Online settings leave real gaps — domain authentication that isn't fully enforced, anti-phishing policies left at their permissive defaults, and quarantined mail nobody's checking.
The controls that actually stop phishing
Email security gaps almost always trace back to one of these being missing or left at default settings, not to a novel attack technique.
- Authenticate your domain. SPF, DKIM, and DMARC configured and enforced, not just published as records nobody's checking.
- Tighten anti-phishing policies. Impersonation protection and spoof intelligence tuned beyond Microsoft's default thresholds.
- Enable Safe Links and Safe Attachments. Real-time scanning of links and attachments at the moment they're opened, not just at delivery.
- Review quarantine regularly. A process for checking what's been caught, so legitimate mail doesn't sit blocked for days.
Why DMARC enforcement matters
Publishing a DMARC record without setting it to reject or quarantine failing mail is close to publishing nothing — it's the enforcement policy that actually stops spoofed mail impersonating your domain.
The most common mistake
Leaving anti-phishing and impersonation policies at Microsoft's default settings, which catch obvious spam but miss more targeted business email compromise attempts.
Email Security Checklist
| Task | Status |
|---|---|
| Publish and enforce SPF, DKIM, and DMARC | ☐ |
| Set DMARC policy to quarantine or reject, not monitor-only | ☐ |
| Tune anti-phishing and impersonation protection policies | ☐ |
| Enable Safe Links and Safe Attachments | ☐ |
| Set up a regular quarantine review process | ☐ |
| Train staff to recognize and report suspected phishing | ☐ |
Frequently Asked Questions
What's the difference between SPF, DKIM, and DMARC?
SPF lists which servers can send mail for your domain, DKIM signs messages to prove they weren't altered in transit, and DMARC tells receiving servers what to do when SPF or DKIM checks fail.
Will stricter email security block legitimate mail?
Poorly tuned policies can, which is why DMARC and anti-phishing thresholds are usually rolled out in monitor mode first, then tightened once false positives are ruled out.
Do Safe Links and Safe Attachments require a specific Microsoft 365 plan?
They're part of Defender for Office 365, which comes with certain Business and Enterprise plans or can be added as a standalone license.
How often should quarantined email be reviewed?
Regularly enough that legitimate mail isn't stuck for days — weekly at minimum, with a clear process for users to request release.
Related Guides
Conclusion
Improving Microsoft 365 email security mostly comes down to finishing what's already half-configured — enforcing DMARC instead of just publishing it, tuning anti-phishing policy past its defaults, and actually checking quarantine. None of it requires a plan upgrade; it requires someone to go in and turn the dials.
Rua Technologies configures domain authentication, anti-phishing policy, and Defender protections as part of every Microsoft 365 email setup we manage.
