Exchange Online backups fail with 'Failed to prepare list of folders' when EWS access is disabled

Exchange Online backups fail with 'Failed to prepare list of folders' when EWS access is disabled

Machine-distilled from a resolved support ticket on 2026-09-28. Source ref: 1bc523fb1c5f. Verify before relying on it.

Applies to: CloudAlly backup of Microsoft 365 Exchange Online mailboxes, for releases where the Exchange backup still relies on Exchange Web Services (EWS) and has not yet moved to Microsoft Graph API.

Symptom: Some Exchange Online mailbox backups fail with the error 'Failed to prepare list of folders', while the overall backup status in the portal may still appear successful.

Cause: The backup service uses Exchange Web Services (EWS) to enumerate mailbox folders. If EWS is disabled or restricted, folder enumeration fails. EWS can be off at mailbox level (EWSEnabled set to false) or restricted at tenant level, or limited by Microsoft ahead of the planned EWS deprecation. The account used to authenticate the backup is the key one to check, not only the individual backed up mailboxes.

Resolution: 1. Connect to Exchange Online PowerShell with an administrative account: Connect-ExchangeOnline -UserPrincipalName <admin UPN>.

2. Check EWS on the account used to authenticate the backup, and on any affected mailbox: Get-CASMailbox -Identity <mailbox UPN> | fl EWSEnabled.

3. If the result is False, enable it: Set-CASMailbox -Identity <mailbox UPN> -EWSEnabled $true.

4. Confirm EWS is not blocked at tenant level (organisation configuration).

5. Allow up to 24 hours for the change to propagate, then check the next scheduled backup.

6. If backups still fail, collect evidence that EWS is enabled at both tenant and mailbox level and escalate to the vendor.