1. Log into ISPConfig
2. Go to Email tab
3. On the left on Global Filters select the “Content Filter”
4. Press on “Add new Content Filter”
5. Make sure that it uses “Header Filter”
6. In the Regexp Pattern input enter for example:
/<.+?@.+?\.xyz>/7. In the Data input box you can enter some message. I use like: “No .xyz mail allowed here”
8. For the Action use “Reject” or “Discard”
9. Save
Credit: Sjau