You can choose anything you want as your custom email domain, as long as it follows the format of username@subdomain.yourdomain.com.
Here are some examples of what you can use for your username and subdomain:
- hello@mail.yourdomain.com
- hi@hive.yourdomain.com
- yourcompany@mail.yourdomain.com
- [anything you choose]@[anything you choose].yourdomain.com