If you are getting spam emails from the same domain but with random email users, you can block that domain from your DirectAdmin control panel.
1. Login to your DirectAdmin Account.
2. In the E-mail Manager section, click on the SPAM Filters icon or type SPAM Filters in the navigation filter box and the option will appear. Click on it.
3. Under Block By from the drop-down menu, choose the Domain option.
4. In Value, type the domain name without http or www.
5. Click the Block button.
If you want to block more domains, just repeat this tutorial from steps 3 to 5.