De4sec
De4sec Technology
de4sec.technology
๐ŸŒ AU + KE

Email Security Guide

A comprehensive guide to securing business email โ€” SPF, DKIM, DMARC, Defender for Office 365, and protection against phishing, spoofing, and BEC.

Prepared by
De4sec Technology
Contact
support@de4sec.technology
Edition
2026 ยท March
CONFIDENTIAL ยท FOR CLIENT USE ONLY
Contents
  1. Why Email Is the Top Attack Vector
  2. Email Authentication: SPF, DKIM, DMARC
  3. Microsoft Defender for Office 365
  4. Mailbox Security Configuration
  5. Business Email Compromise Prevention
  6. De4sec Email Security Service
01

Why Email Is the Top Attack Vector

Email remains the primary entry point for cyber attacks โ€” phishing, malware delivery, business email compromise, and domain spoofing all begin with email. Securing email is not optional; it is the foundation of any business security posture.

The ACSC reports that over 60% of cyber incidents reported by Australian businesses involve email as the initial attack vector.

02

Email Authentication: SPF, DKIM, DMARC

StandardFunctionWithout it
SPFAuthorises which servers can send on your behalfDomain spoofing is trivial
DKIMCryptographic signature on outbound emailEmail can be forged or modified in transit
DMARCEnforcement policy for failed authenticationEven with SPF+DKIM, no action taken on spoofed email

Target: DMARC p=reject. This eliminates domain spoofing โ€” attackers cannot send email appearing to come from your domain. Configuration takes 2โ€“4 hours; the impact is immediate and permanent.

03

Microsoft Defender for Office 365

Safe Links

Rewrites all URLs in emails. At click-time, the URL is scanned in real-time โ€” even if the destination becomes malicious after delivery. This is critical because attackers often send clean links and weaponise them later.

Safe Attachments

Detonates attachments in an isolated sandbox before delivery. Malicious attachments are blocked before the user sees them โ€” even unknown malware types with no signature.

Anti-Phishing Policies

Protects specific users (CEO, CFO, IT admin) from impersonation. Alerts when email appears to be from a protected user but originates externally.

04

Mailbox Security Configuration

โœ“Disable email forwarding rules by default โ€” attackers set up auto-forward rules after compromising an account
โœ“Audit logging enabled on all mailboxes โ€” detect suspicious access, forwarding rule creation, mass deletion
โœ“External sender warning banners โ€” visually mark external email so staff can identify unexpected senders
โœ“Shared mailbox access via Entra ID groups โ€” not individual account assignment
โœ“Mail transport rules โ€” block certain attachment types (exe, bat, vbs) from external senders
05

Business Email Compromise Prevention

BEC involves impersonating executives, suppliers, or IT staff to redirect payments or obtain sensitive information. Technical controls reduce risk but cannot eliminate it โ€” process controls are essential.

โœ“Payment verification procedure: any payment request received by email requires phone confirmation to a pre-known number
โœ“Supplier change verification: any request to update bank details triggers a call to the existing supplier contact
โœ“Executive travel alerts: when the CEO is travelling, finance is alerted that payment request emails may increase
โœ“Dual approval for large transactions: any payment above a threshold requires two approvers
06

De4sec Email Security Service

Email Authentication
SPF, DKIM, DMARC configuration, testing, and ongoing monitoring. DMARC reporting reviewed monthly.
Defender for Office 365
Safe Links, Safe Attachments, anti-phishing policy configuration. Tailored to reduce false positives.
Security Awareness
Staff training on identifying phishing and BEC. Simulated phishing exercises. Click-rate tracking.
Incident Response
When a phishing email is clicked or account compromised: containment, credential reset, investigation, post-incident report.
// NEXT STEP

Ready to implement this?

De4sec provides hands-on implementation. Book a free discovery call โ€” we assess your environment at no cost.

Book a Free Discovery Call โ†’de4sec.technology
De4sec
ยฉ 2026 DE4SEC TECHNOLOGY.