Appian Cloud – Receive Message Email Event Not Receiving Emails from External Domain

We are facing an issue with the Receive Message Event in an Appian Cloud application.

Our application sends approval request emails to both internal and external users. When internal users reply to the approval email, the response is successfully received and triggers the Appian process model.

However, when users from an external email domain reply to the same approval email, their responses are not reaching the Appian process model.

Environment Details

• Appian Environment: Appian Cloud
• Event Type: Receive Message Event
• Message Type: Email
• Public Events: Enabled
• Process Model Email Address: processmodeluuid*************@apn.***.com (custom domain)

Troubleshooting Performed

• Verified that the Receive Message Event is configured to listen for Email.
• Verified that Public Events is enabled in the process model properties.
• Confirmed that the process model email address is correctly configured.
• Verified that internal email responses are successfully triggering the process model

Questions for the Community

• Has anyone encountered a similar issue where Appian Cloud Receive Message Events receive emails from internal domains but not from specific external domains?
• Are there any additional Appian Cloud configurations required to receive emails from external Domain
• Is there any Appian-side configuration or restriction that could prevent emails from specific external domains from reaching the Receive Message Event?
• Are there any additional logs or troubleshooting steps available on the Appian Cloud side to identify why these emails are not reaching the environment?
• Does appian allow response from external domain response.

Any suggestions or guidance would be greatly appreciated.

Thank you!