Mimecast (Sync & Recover, Continuity): ready for the EWS shutdown?

Short answer: Mimecast says its EWS apps (Sync & Recover, Continuity) must be on your tenant's EWS allow list, or they stop working once EWS is blocked. Add Mimecast's app IDs from your tenant to the list and follow Mimecast's own article for the Graph timeline.

✖ Still needs EWS · vendor statements checked 26 Oct 2026

Deadline: Exchange Online blocks EWS by default from October 2026 (tenants are switched to EwsEnabled False, rollout to late April 2027) and turns it off for good on 1 April 2027, with no exceptions. Exchange Server on-premises is not affected.

Status

Needs the allow list (Mimecast support article 55196651141395). The apps are registered per tenant with names starting "Mimecast".

This vendor registers its own app in each tenant, so the app ID is yours. Find it in the EWS usage report, or run the free script with -ResolveNames.

What to do

  1. Find the Mimecast app IDs (Entra → Enterprise applications, search "Mimecast", or the script with -ResolveNames).
  2. Add them to the allow list, merged with your current IDs.
  3. Check Mimecast's article for any version or permission it expects before you change the list.

More than one app? Run the free read-only script: it names every app still calling EWS in your tenant.

Allow-list it safely

Setting EwsAllowedAppIDs replaces the whole list, so adding one ID on its own removes every other app. Read the current list first, then write the merged list:

Get-OrganizationConfig -RetrieveEwsOperationAccessPolicy | Format-List EwsEnabled,EwsAllowedAppIDs
Set-OrganizationConfig -EwsEnabled $true -EwsAllowedAppIDs "<current IDs>,<new ID>"

Changes to the list take up to 24 hours. A blocked call returns HTTP 403 with the header X-EWS-Policy-Reason. The free script prints the merged command for every app in use (it never runs it).

Sources

MSP or IT partner? The M365 Readiness Pack runs this EWS check, the SMTP AUTH check and SPF/DKIM/DMARC for every domain once per client tenant, and writes a client-ready report with your logo, next steps and an email to send the client. Read-only, works with GDAP.

See the pack (from $49)
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