Short answer
Mimecast adds an independent, specialist email security layer in front of Microsoft 365, with its own threat intelligence, sandboxing and URL rewriting, plus continuity if Microsoft 365 itself is down. FirstNet also runs the tuning, threat workflows and DMARC programme for you.
In detail
The value is defence in depth plus an operating model:
- A separate filtering layer, so one vendor's detection gap is not your only line of defence
- Impersonation and business email compromise protection, including protection for internal email after an account is compromised (Resilience tier and above)
- Email continuity through Outlook, web and mobile apps during a Microsoft 365 outage, where contracted
- Archiving with retention from 30 days to 99 years, where contracted
- FirstNet operating the platform: baseline policies, ongoing tuning of allow and block rules, quarantine and release workflows, threat triage and monthly reporting
Microsoft's own controls remain the baseline. Mimecast complements them rather than replacing your mail platform, and it integrates with Entra ID, SIEM and ticketing tools.
If your team lacks the time to tune and investigate email threats every day, this managed layer takes that work off them.
Source: FirstNet Email Security & Threat Protection service page →
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