Steps to resolve “MailboxNotEnabledForRESTAPI” error:
1. Ensure the User’s Mailbox is Active:
• Make sure that the user associated with the mailbox you’re trying to access has an active Exchange Online mailbox.
• This can happen if the mailbox is not yet fully provisioned, or if the user is missing an Exchange Online license.
Check Exchange Online License:
• Go to Microsoft 365 Admin Center.
• Ensure that the user has the appropriate Exchange Online license assigned.
2. Check that the mailbox is not disabled:
• If the mailbox is disabled, it won’t respond to REST API calls.
• Use PowerShell to check the mailbox status:
Get-Mailbox -Identity <user_email_address>
• Ensure that the mailbox is enabled.
3. Enable REST API for the Mailbox:
• In some cases, a mailbox may not have the required permissions to interact with REST APIs.
• To enable a mailbox for REST API access, run the following PowerShell command using the Exchange Online PowerShell module:
Set-CASMailbox -Identity <user_email_address> -EwsEnabled $true
4. Grant Required API Permissions to the Azure AD App:
• Go back to your Azure AD App Registration in the Azure portal.
• In the API permissions section, you need to add and grant the following permissions to your app:
• Microsoft Graph API or Exchange API.
• The necessary permissions for sending emails via SMTP include:
• Mail.Send
• Mail.ReadWrite
• Mail.Read
• Make sure these permissions are granted as Application Permissions (for service-to-service communication) and not as Delegated Permissions (unless you’re authenticating on behalf of a user).
After adding the permissions:
• Click on “Grant admin consent” for the permissions.
5. Configure Application Authentication for SMTP (OAuth2):
• For the SMTP connection, you need to ensure that OAuth2 authentication is set up properly.
• When using an application in Azure to authenticate with Microsoft 365 for SMTP, you should:
• Use Client ID and Client Secret from your registered app.
• Use the token endpoint to authenticate the app and retrieve an access token.
• The SMTP server address for Microsoft 365 is smtp.office365.com with port 587 (TLS).
6. Check the Permissions of the Service Account:
• The service account (or the mailbox being used to send the email) must have permissions to send mail on behalf of itself or another mailbox if needed.
• If you’re using a service account, make sure it has the Send As or Send on Behalf permissions if necessary.
• You can check or assign these permissions via Exchange Online PowerShell using:
Add-MailboxPermission -Identity <target_mailbox> -User <service_account> -AccessRights SendAs
7. Enable Modern Authentication (OAuth) for SMTP in Microsoft 365:
• SMTP with OAuth requires Modern Authentication to be enabled on your tenant.
• Verify if Modern Authentication is enabled by running the following PowerShell command:
Get-OrganizationConfig | ft OAuth2ClientProfileEnabled
• If it returns False, enable it:
Set-OrganizationConfig -OAuth2ClientProfileEnabled $true
8. Use the Correct SMTP Settings:
• Ensure that the SMTP settings in your application match the correct ones for Microsoft 365:
• Server: smtp.office365.com
• Port: 587 (TLS)
• Authentication: OAuth2 with the token retrieved from Azure AD.
After Registration Process:
1. Add API Permissions in Azure Portal:
• Go to Azure Active Directory > App registrations > Your app.
• In the API Permissions tab, add Microsoft Graph or Exchange API permissions related to sending emails (as mentioned in Step 4).
2. Generate a Client Secret:
• Go to Certificates & Secrets in your app registration.
• Generate a Client Secret and note it down (you will need it to authenticate).
3. Set Up OAuth2 in Your Application:
• Your application must use the Client ID, Client Secret, and Tenant ID to authenticate via OAuth2.
• The SMTP service will authenticate using the access token retrieved via OAuth.
Regards,
Rushikesh Vartak
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