Symptoms

The support@company.com mail channel is configured, but when adding an internal comment, the email is sent from support@company.com instead of support+internal@company.com.

The mail is managed by Google (GSuite / Google Workspace / Gmail).

The following is visible in the headers of the mail message:

From: Support <support@company.com>
X-Google-Original-From: Support <support+internal@company.com>
Subject: Test delivery
To: recepient@other-company.com
Reply-To: Support <support+internal@company.com>

Solution

It is necessary to configure an additional address on the Google Mail account side

1. Log in to the mail application under the support@company.com account
2. Click on the icon
3. Click Settings > Accounts and Import
4. Scroll down to the Send mail as section and click Add another email address
   
5. Add the address with the "+internal" suffix before the @, i.e., for our example, it would be support+internal@company.com
6. Uncheck the Use as alias checkbox and click Next
   

Reason

Since 2025, Google has changed the mechanism for handling aliases of the "+alias" type and now substitutes such aliases with the original address by default, saving the original From field in the X-Google-Original-From header