Mimecast Outlook Plugin Unloading: Keeping the Add-In Active

Fixing an Inactive Mimecast Outlook Add-In That Keeps Being Unloaded

If Outlook keeps disabling the Mimecast for Outlook add-in on every restart — showing it as an inactive or disabled application add-in even though you keep re-enabling it — this is almost always because Outlook itself flagged it as slow or unresponsive at some point and automatically blacklisted it, which it does silently to protect startup performance. To clear this and let the add-in load normally again:

• Fully close Outlook.
• Uninstall the Mimecast for Outlook software from Programs and Features.
• While still logged in as the affected user, open the Registry Editor and go to HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook\Resiliency (replace <version> with your Office version number, e.g. 16.0).
• Delete the entries under both the DisabledItems and CrashingAddinList subkeys — these are what’s actually recording the add-in as unstable and preventing it from loading.

Reinstall the Mimecast for Outlook software fresh, then open Outlook and confirm the add-in loads and stays enabled. Back up the Resiliency key before deleting anything if you want to be able to restore it, though these entries rebuild automatically as Outlook needs them.

Still Applies to Classic Outlook — Not New Outlook (Updated for 2026)

This fix is still accurate for classic Outlook (Microsoft 365 Apps, Outlook 2021/2024) — the Resiliency registry mechanism is unchanged. It does not apply to the newer unified “new Outlook” app, though: new Outlook doesn’t support classic COM add-ins like the Mimecast plugin at all, only web add-ins managed centrally through the Microsoft 365 Apps admin center, so there’s no registry key to clear if that’s what you’re running instead.