Indicative results from Mycelium 365 Mimecast to Microsoft Defender for Office 365 consolidations across 50–250 seat Australian environments.
Mimecast subscription removed where Defender for Office 365 is already owned in Business Premium or E5
Policy audit, Defender tuning, parallel run, MX cutover and decommissioning
Mimecast retained as a fallback route through the agreed window
Investigation consolidated into the Microsoft Defender portal with AIR automation
Savings are indicative and depend on your current Mimecast contract, seat count and Microsoft 365 licensing tier.
Email security changes are only defensible with evidence. During the parallel run both platforms scan the same live mail, which gives a like-for-like detection comparison rather than a vendor claim. We baseline these metrics on Mimecast, repeat them 30 days after cutover, and include the comparison in the project close-out pack.
| Metric | Mimecast baseline | Post-Defender target | How it is measured |
|---|---|---|---|
| Malicious mail blocked | Baseline detection count | Parity or better, verified in parallel run | Threat Explorer vs Mimecast rejection logs, same mail sample |
| False positives released by users | Typically 2–5% of quarantined mail | Under 1% after 30 days of tuning | Quarantine release reporting in the Defender portal |
| Mean time to triage a reported phish | 30–60 minutes, manual | Under 10 minutes with AIR playbooks | Automated investigation and response timelines |
| Coverage beyond email | Email only | Teams, SharePoint and OneDrive included | Safe Links and Safe Attachments policy scope |
| Annual email security spend | Mimecast subscription plus admin overhead | Included in existing Microsoft 365 licensing | Renewal invoice versus post-consolidation run rate |
| Security admin console switches per incident | 2–3 portals | 1 portal | Incident handling runbook review |
1. Silent policy loss. Years of accumulated Mimecast content examination rules, bypass entries and impersonation lists rarely exist as documentation. We export the full policy set and build a line-by-line mapping to Defender anti-phishing, anti-spam, Safe Links, Safe Attachments and Exchange transport rules before anything is switched.
2. Archive and legal hold obligations. If journaling or legal hold is active, the archive question must be answered before cutover, not after. We agree in writing whether content moves into Microsoft Purview for native retention and eDiscovery, or stays read-only in Mimecast for the retention period, and we cost both paths.
3. Detection regression. Switching security controls without evidence is a governance failure. The parallel run scans identical production mail through both platforms so any gap is visible and tuned out while Mimecast is still in front of your mail flow.
4. End-user workflow shock. Quarantine digests and release workflows look different in Defender. We run comms and short enablement sessions ahead of cutover so the service desk is not absorbing "where is my email" tickets during the first week.
5. Paying twice. Consolidation only banks a saving if the Mimecast contract is exited on schedule. We plan the cutover against your renewal and notice period so the licence is cancelled the month it becomes redundant.
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.
