Mimecast vs Microsoft Defender for Office 365: do you need both?
Microsoft Defender for Office 365 is good, it is included in the licences many organisations already own, and for some of them it is enough. Mimecast is the layer most UK enterprises put in front of it, and for good reasons. This guide compares the two on the things that decide the outcome, says plainly when Defender alone is the right answer, and explains why we usually recommend Mimecast where the risk is real. C4C is an accredited Mimecast partner, and that is disclosed up front so the comparison can be judged on its reasons.
If your email runs on Microsoft 365, you already have Defender for Office 365 in some form, and the honest first question is whether it is switched on at the plan you are paying for. A lot of the estates we look at are running the baseline with the advanced plan unlicensed or untuned, and no comparison is fair until that is fixed. Our guide on whether built in email security is enough covers that step. This guide assumes you have done it, and asks the next question: with Defender fully on, do you still need Mimecast?
The short version is that Defender and Mimecast are not the same kind of thing. Defender is Microsoft protecting Microsoft, from inside the tenant. Mimecast is an independent layer that sits in front of the tenant, sees mail before Microsoft does, keeps working when Microsoft does not, and covers estates that are not only Microsoft. Whether that difference is worth paying for depends on who you are and what an attacker stands to gain.
What each one actually is
Microsoft Defender for Office 365 comes in two plans on top of the Exchange Online Protection baseline every mailbox gets. Plan 1 adds Safe Links, Safe Attachments and anti phishing with impersonation protection. Plan 2 adds automated investigation and response, threat hunting, attack simulation training and the zero hour purge that pulls a message back after delivery. It is included in Microsoft 365 E5 and available as an add on to other plans. Run properly at Plan 2, it is a serious email security product, and Microsoft sees more global mail than anyone, which helps it spot new campaigns early.
Mimecast Advanced Email Security is a cloud email security platform that sits alongside or in front of Microsoft 365 and Google Workspace. It rewrites and scans links at the moment they are clicked, detonates attachments before delivery, scores impersonation and payment fraud across sender, domain, display name and the language of the request, takes your domains to DMARC enforcement and watches for lookalike domains that trade on your brand, provides a working mailbox through a Microsoft outage, and delivers awareness training aimed at the people who actually click. It is licensed per user and priced on quote. Our Mimecast Advanced Email Security page covers what it does in more detail.
Head to head
The rows below are the things that decide the outcome in practice. Where Defender genuinely holds its own we say so.
| What matters | Defender for Office 365 | Mimecast |
|---|---|---|
| Where it sits | Inside the Microsoft 365 tenant. Protects Microsoft from Microsoft. | Independent of the tenant. Sees mail before Microsoft does and is not affected by a tenant compromise. |
| Commodity phishing and malware | Strong, especially at Plan 2. Benefits from Microsoft’s global view of mail. | Strong. Click time link scanning and attachment sandboxing on every message. Broadly comparable on the everyday volume. |
| Impersonation and business email compromise | Impersonation protection covers named users and domains once configured. Weaker on the carefully written, payload free request. | Scores sender, domain, display name, sender history and the language of the request together. This is where most of the real losses sit and where Mimecast has the edge. |
| After a message is delivered | Zero hour purge and automated investigation at Plan 2 can pull a message back. | Removes a delivered message from every affected mailbox. Similar outcome, and it still works if the tenant is the problem. |
| When Microsoft 365 is down | None. Defender is part of the platform that is unavailable. | Continuity gives users a working mailbox through the outage. Defender has no answer to this by design. |
| DMARC and brand protection | No DMARC reporting or analysis tooling. Nothing for lookalike domains outside your estate. | DMARC reporting to take domains to enforcement safely, plus detection of lookalike domains and cloned sites using your name. |
| Awareness training | Attack simulation training at Plan 2. Capable, Microsoft centric. | Short, regular training and simulation targeted at the users who click, linked to the platform’s risk view. Comparable, and the targeting is the useful part. |
| Mixed estates and Google Workspace | Microsoft 365 only. | Microsoft 365 and Google Workspace, one policy and one view across both. |
| Licensing | Included in E5, add on elsewhere. If you already own E5, the incremental cost is zero. | Per user subscription on quote. Costs money you may not be spending today. |
| Effort to run well | Needs tuning and an owner. Left on defaults it underperforms its own plan. | Managed policy, and C4C implements and supports it as an accredited partner. |
When Defender alone is the right answer
We will point away from Mimecast here, because it is often correct. If you are all in on Microsoft with no Google Workspace in the estate, you already own E5 or are willing to license Defender at Plan 2, someone owns it day to day and has tuned the impersonation and phishing policies, you are not a high value or heavily regulated target, and your exposure to payment fraud is modest, then Defender is probably enough. The incremental protection Mimecast adds in that situation may be against threats you do not realistically face, and a second product nobody has time to run adds cost without adding safety.
The one thing Defender cannot do in that scenario is keep your email working when Microsoft 365 is down. If a day without email is a business continuity problem for you, that alone can justify the Mimecast layer. If it is an inconvenience, it does not.
Two questions. Is Defender licensed at the plan you think it is, switched on, and owned by someone. And do the gaps that remain, payload free impersonation, continuity through an outage, DMARC and brand, a mixed estate, match the threats that would actually hurt you. If the first answer is no, fix that before you buy anything. If the second is no, you may already be covered.
When Mimecast earns its place, and why we usually recommend it
For most of the organisations we work with, the gaps in the table map directly onto the threats that do real damage, and that is why Mimecast is the layer we recommend more often than not. Business email compromise carries no payload, so it slips past anything looking for one, and the losses are measured in bank transfers rather than infections. Mimecast scores the request itself, which is the right place to catch it. An independent layer keeps working when the tenant is compromised or unavailable, which matters the moment resilience is part of the requirement. DMARC enforcement and brand protection stop your own name being used against your customers and suppliers, which Defender does not attempt. And if any part of the estate is on Google Workspace, one policy across both is worth more than two half configured ones.
There is also the operational point. Defender at Plan 2 is capable, and it is capable in the hands of a team that tunes it. Mimecast arrives with managed policy, and with C4C implementing and supporting it, the protection is in place in weeks rather than becoming a project the security team never quite finishes. For a lot of UK enterprises, that difference is the real one.
Running both is a common and sensible pattern: Mimecast in front, handling the targeted threats, continuity, DMARC and brand, with Defender’s baseline left on behind it. What we do not recommend is running two full email security platforms in parallel and trying to keep both tuned. Pick the layer, keep the baseline.
Where C4C stands
C4C Group is an accredited UK Mimecast partner and reseller. We supply, implement and support Mimecast, and we tell clients when Defender alone is the right answer, which happens and which is why the recommendation means something when we make it. Our independence is in the recommendation, our partnership is in the delivery and support. The fastest way to find out which side of the line you are on is the free email security assessment, which gives an honest read on where your current protection is thin.
Not sure whether Defender is enough for you?
Send us your Microsoft 365 plan, rough user count and whether any of the estate is on Google Workspace. We will tell you honestly whether Defender properly run covers your risk, or whether Mimecast earns its place, and give you a like for like quote if it does. We will tell you if it does not.
Prefer email? Reach us directly at hello@c4cgroup.co.uk.
Frequently asked questions
Is Mimecast better than Microsoft Defender for Office 365?
Neither is simply better. Defender is Microsoft protecting the Microsoft tenant from inside it, and at Plan 2, properly tuned, it handles everyday phishing and malware well. Mimecast is an independent layer that is stronger on payload free impersonation and business email compromise, keeps email running through a Microsoft outage, covers DMARC and brand protection, and spans Google Workspace as well. Which you need depends on your risk, your estate and who will run it.
Do I need Mimecast if I already have Microsoft 365 E5?
Not automatically. If you own E5, Defender at Plan 2 is already yours and the incremental case for Mimecast is weaker, provided someone actually tunes and owns Defender. Mimecast still earns its place if you are exposed to payment fraud, need email to keep working through a Microsoft outage, want DMARC enforcement and brand protection, or run a mixed estate. C4C will say which applies to you.
Can Mimecast and Defender for Office 365 run together?
Yes, and it is a common pattern. Mimecast sits in front, handling targeted threats, continuity, DMARC and brand, and Defender’s baseline stays on behind it. What you should avoid is running two full email security platforms in parallel and keeping both tuned. Choose the layer and keep the baseline.
What does Mimecast do that Defender cannot?
Three things stand out. Continuity: a working mailbox when Microsoft 365 is down, which Defender cannot offer because it is part of the platform that is unavailable. DMARC reporting to enforcement and detection of lookalike domains using your brand. And one policy across Microsoft 365 and Google Workspace. Mimecast also scores impersonation across more signals, which is where most real losses sit.
Which is cheaper, Mimecast or Defender?
If you already own E5, Defender costs nothing extra and Mimecast is a new per user subscription. If you would be adding Defender Plan 2 as an add on, the comparison is closer. Mimecast is priced on quote, so the honest answer for your estate needs your user count and plan. C4C gives like for like quotes and says what is negotiable.
Is C4C independent if it partners with Mimecast?
C4C is an accredited Mimecast partner and reseller, disclosed on every page that recommends it. The test of independence is whether we will point you away from it, and we do: for organisations all in on Microsoft with Defender properly run, we say Defender is enough. Mimecast is a frequent recommendation because it fits a common set of risks well, not because it is the answer to every question.