Outgoing Blacklist Filtering Rules
In the Outgoing Blacklist Filtering Rules page you can view all blacklist filtering rules that have been set up for your domain's outgoing mail and you can also add new ones. Outgoing mail that matches any of the rules will always be blocked.
The rules are based on Python's regular expression (regex) syntax. (For more information on regular expressions, see regex101.com).
You can access this page at the Admin Level and the Domain Level.