Migrating from Mimecast to Microsoft Defender for Office 365 consolidates email security into your existing Microsoft 365 licensing — Mycelium 365 handles the full Mimecast to Defender migration for organisations, replacing third-party email gateways with native Microsoft protection across mail, Teams, SharePoint and OneDrive.
Most organisations on Microsoft 365 Business Premium or E5 already own Defender for Office 365 Plan 1 or 2 as part of their existing licensing — paying for Mimecast on top is effectively paying for the same control twice. Consolidating onto Defender removes that duplication, simplifies the security stack and improves signal sharing across Defender for Endpoint, Entra ID Identity Protection and Intune. Safe Links and Safe Attachments extend protection into Teams, SharePoint and OneDrive, automated investigation and response (AIR) cuts triage time, and the single Microsoft 365 Defender portal gives admins one place to investigate incidents rather than pivoting between vendor consoles. The architecture also maps cleanly to ACSC Essential Eight controls — application control, MFA, admin privilege restriction — without bolting on third-party tooling, and Australian data residency is preserved through Microsoft's local regions.
Every Mycelium 365 migration starts with a policy audit — full inventory of Mimecast inbound and outbound policies, content examination rules, URL protect settings, attachment policies, mail flow rules, archive retention and end-user release workflows — so nothing is dropped at cutover. From there we move to Defender tuning: Safe Links, Safe Attachments, anti-phishing with impersonation protection, anti-spam and preset security policies configured to match your risk profile.
A parallel run is established with both platforms scanning live mail, so detections can be validated against real production traffic before any change to mail routing. Archive handling is agreed up front — Mimecast archive content can be migrated into Microsoft Purview for native retention and eDiscovery, or retained read-only in Mimecast for the agreed period.
The cutover updates DNS MX records and Exchange Online connectors to route mail through Defender, with Mimecast kept live as a fallback through the agreed window. Finally, we handle decommissioning — Mimecast connectors removed, licences cancelled and the Microsoft 365 Defender portal handed over to your team with documentation and runbooks.
Most Australian Mimecast to Defender migrations complete in 4 to 6 weeks end-to-end. A typical 100-user organisation with a standard policy footprint lands inside 4 weeks including policy audit, Defender tuning, parallel run, cutover and 30 days of hypercare.
Timeline is driven by the size of the Mimecast archive, the complexity of existing policies and mail flow rules, whether journaling or legal hold is in place, and how much end-user communication is required around quarantine and release workflow changes.
Mycelium 365 includes a 30-day hypercare period after cutover — daily quarantine triage, policy tuning against real production detections, and end-user enablement on the new release workflow — before the environment transitions into ongoing managed services.
A structured four-phase approach to move from Mimecast to Microsoft Defender.
We inventory every Mimecast policy, mail flow rule, archive retention setting and end-user release workflow so nothing is missed at cutover.
Configure Defender for Office 365 — Safe Links, Safe Attachments, anti-phishing, impersonation protection and preset security policies — to match your risk profile.
Run both platforms side by side, validate detections on real production mail, then cut DNS and connectors over to Defender in a controlled window.
Decommission Mimecast, hand over the Defender portal to your team and run 30 days of hypercare with daily triage and tuning.
Every layer of your Mimecast deployment, mapped and rebuilt natively in Microsoft Defender.
Mimecast connectors, transport rules and routing replaced with Defender-native mail flow and Exchange Online transport rules.
Anti-spam, anti-malware, Safe Links, Safe Attachments and anti-phishing policies rebuilt natively in Defender for Office 365.
Mimecast archive content migrated into Microsoft Purview or retained read-only, with retention, eDiscovery and legal hold preserved.
End-user quarantine, admin review and reporting consolidated into the Microsoft 365 Defender portal.
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