You can choose anything you want as your custom email domain, as long as it follows the format of username@subdomain.yourdomain.com.
Note: "subdomain" is not what you would use, but is simply a placeholder for the subdomain you choose
Here are some examples of what you can use for your username and subdomain:
hello@mail.yourdomain.com
hi@newsletter.yourdomain.com
[anything you choose]@[anything you choose].yourdomain.com
Example: If your company is Hightown Media, and your website/email domain is "HightownMedia.com" then the format of your subdomain could be hello@mail.hightownmedia.com