This is so simple to do, but I have to look it up every time I need it (not something that comes up regularly!);
When configuring a development server, you may find you have a need to ensure that emails will not be sent to any domain except those you explicitly permit (for example if you're using real-world data to do some testing, do you want to send all those users irrelevant emails?).
This documentation details how to configure Postfix on a Linux server to disregard any mail sent to domains that are not explicitly permitted.