This tutorial will show you how to mark all incoming messages from external domains as an external sender.
Step 1: Click on the Settings icon in the top navigation bar
Step 2: Click on Content Filtering
Step 3: Click the New button
Step 4: Give this filter a name, in our example, we named it External Sender
Step 5: Click on + New Condition
Step 6: Set the following conditions:
1. Condition Type: From Address
2. Field: From specific domains
3. Comparison: Does Not Match
4. From specific domains (one per line): enter your domain name
Click Save
Step 7: Click on + New Action
Step 8: Set the following action:
Action: Add Text to Subject
Text to Add: in this example, we put [EXTERNAL], but you can put anything you want
Click Save
Step 9: Click Save