[Email Security] MX/Inline note (#22436)

Author: Maddy-Cloudflare

src/content/docs/cloudflare-one/email-security/setup/pre-delivery-deployment/mx-inline-deployment-setup.mdx

@@ -17,6 +17,10 @@ To set up MX/Inline:
 4. Select **MX/Inline**.
 5. To start the MX/Inline configuration, you will need to have completed the prerequisite setup on your email provider's platform. Once you have completed this step, select **I confirm that I have completed all the necessary requirements**. Then, select **Start configuration**.
 
+:::[note]
+You can only onboard one domain at a time.
+:::
+
 If you have verified zones on Cloudflare, continue with the following steps:
 
 1. **Connect a domain**: Select your domain. Then, select **Continue**.