Yes you can. Since Mailman is access via the web and email, a domain name is needed. So if you have control of a domain name that is NOT being used with another web or email service, then you can use it with us. If it is being used with another hosting service, then we recommend the use of a subdomain (e.g. list.yourdomain.com) that is based off of the domain name that you wish to use. It is also possible to use a subdomain for web access and a domain name for email, as long as that domain name is NOT being used with another email provider.
Most of our clients uses a subdomain with our Mailman service.