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Email migration
From
Mimecast
→
To
Microsoft Defender for Office 365

Mimecast to Defender for Office 365 migration — UK email security consolidation

For UK organisations already on Microsoft 365 — particularly E5 — Mimecast is often a duplicate spend that Defender for Office 365 can absorb. Defender now ships safe attachments, safe links, anti-phishing, anti-spoofing, attack simulation training and advanced hunting all natively in M365. Servnet runs end-to-end Mimecast → Defender migrations including connector cutover, policy migration and Mimecast decommission.

Vendor migration programme — Mimecast source on the left, Microsoft Defender for Office 365 target on the right, with parallel-running data streams converging through a central Servnet cutover hub.
From → To: Mimecast vs Microsoft Defender for Office 365
CURRENTMimecastProduction workloadsLegacy management planeRenewal due / EoSServnetparallel-running migrationTARGETMicrosoft Defender for Office 365Production workloadsModern management planeStrategic 5-yr position
Typical outcomes

What good looks like after a Mimecast → Microsoft Defender for Office 365 migration

Mimecast spend
£0

Eliminated entirely (assuming M365 E5 already in place; otherwise modelled vs Mimecast cost).

Migration window
4-8 wk

End-to-end including parallel running + Mimecast decommission.

Phishing detection
Comparable+

Defender matches or exceeds Mimecast at typical UK enterprise scale per independent testing.

Management panes
−1

Mimecast portal removed; email security consolidated into Microsoft Defender / Purview admin centres.

The why

Why UK organisations migrate from Mimecast to Microsoft Defender for Office 365

  • ✓Eliminate Mimecast subscription where Defender is already included in M365 E5
  • ✓Single management plane — Defender XDR connects email security to endpoint + identity + cloud security
  • ✓Native integration with Entra ID, Conditional Access, Microsoft Purview DLP
  • ✓Improved phishing detection via Microsoft's telemetry advantage (hundreds of billions of emails/day)
  • ✓Attack Simulation Training built-in for security awareness
  • ✓Reduce email security spend by 100% of Mimecast bill when M365 E5 already paid for
How we run it

Migration phasing — typical Mimecast → Microsoft Defender for Office 365 programme

Mimecast → Microsoft Defender for Office 365 — programme timeline
W0W1W2W3W4W5W6W7W8Discovery + policy mapping2wDefender configuration2wParallel running2wConnector cutover + Mimecast decommission2wTotal programme: 8 weeks · parallel running throughout
  1. 1

    Discovery + policy mapping

    Weeks 1-2

    Mimecast estate fingerprint (policies, retention, journaling, archiving, attachment policies); Defender policy mapping; M365 licensing reconciliation — confirm E5 entitlements or model upgrade cost.

  2. 2

    Defender configuration

    Weeks 3-4

    Defender policies (Standard / Strict / custom); Safe Links + Safe Attachments; anti-phishing + impersonation; auto-investigation + response (AIR); SIEM forwarding.

  3. 3

    Parallel running

    Weeks 5-6

    Mimecast + Defender both processing email; quarantine + reporting reviewed for parity; user-experience validation.

  4. 4

    Connector cutover + Mimecast decommission

    Weeks 7-8

    MX cutover removes Mimecast as inbound gateway; archive / journal data exported per retention requirements; Mimecast contract terminated.

Included in scope

What Servnet delivers in a Mimecast → Microsoft Defender for Office 365 migration

Defender policy templates

Standard, Strict, or custom policies aligned to your existing Mimecast policy intent.

Safe Links + Safe Attachments

Configured for click-time URL rewriting + detonation chamber attachment scanning.

Anti-phishing + impersonation policies

Sender impersonation, domain impersonation, mailbox intelligence — tuned for your environment.

Archive + journal migration

Existing Mimecast archive + journal data exported to Exchange Online Archive, Purview, or third-party retention platform.

SIEM + ITSM integration

Defender alerts forwarded into Sentinel, Splunk or your existing SIEM.

Attack Simulation Training rollout

Built-in phishing simulation + training platform configured + first campaigns launched.

De-risking the cutover

Top risks + how we mitigate them

⚠️ Mimecast contract has years left
Often the M365 E5 dual-spend justifies early co-termination. We model the cost outcome of either (a) running both until Mimecast expiry, or (b) co-terminating early.
⚠️ Existing Mimecast archive / journal retention is contractually required
Archive data exported to Exchange Online Archive or Microsoft Purview (both included in M365 E5) — retention preserved without Mimecast as the storage platform.
⚠️ Defender phishing detection regression vs Mimecast
Parallel running phase compares quarantine + detection volumes per policy. If Defender misses things Mimecast caught, policies are tuned before cutover. Most modern testing shows Defender at parity or better.
⚠️ Users notice changes in quarantine + release workflow
User-experience walkthroughs + comms + helpdesk training before cutover. Quarantine notifications + release portal are configurable to closely match the Mimecast experience.
Pricing guide rail

Indicative: Mimecast → Defender migrations typically run £8k-£20k professional services. Total programme is usually a substantial net saving (annual Mimecast spend eliminated, no incremental Defender cost when E5 already in place). Talk to us for a sized commercial proposal — we'll quantify the year-1 net saving.

Frequently asked

FAQs — Mimecast → Microsoft Defender for Office 365

What if we're on M365 E3, not E5?

Defender for Office 365 P2 (the Mimecast equivalent) is available as an E3 add-on. We model the E3 + Defender P2 cost vs the existing Mimecast cost — usually still favourable. See our Microsoft licensing guide.

Can Defender match Mimecast for archive + e-discovery?

Exchange Online Archive + Microsoft Purview cover Mimecast's archive + e-discovery use cases. For very specific use cases (immutable journal for FCA-regulated firms, certain legal-hold workflows) we may recommend keeping a Purview eDiscovery add-on or third-party archive platform.

What about DLP?

Microsoft Purview DLP is included in M365 E5 and is functionally competitive with Mimecast's DLP. Often migrated as part of the same workstream.

Should we move from Mimecast to Proofpoint instead?

If you're not committed to the Microsoft stack, Proofpoint remains the most-evaluated independent email security platform. For Microsoft-aligned shops, Defender is the typical winner on cost + consolidation.

Go deeper

Ready to scope your Mimecast → Microsoft Defender for Office 365 migration?

30-minute discovery call with an engineer who's run this migration before. Honest scoping, no sales script.

Book a scoping call →