There is a feature within the Mail component to create new users from inbound mails if they don't already exist. This however doesn't work if you choose to create them as Contacts rather then End Users. This is because the Company relationship on the Contact object is mandatory and there isn't any way to populate this automatically.
A workaround for this is to set a default value for the attribute. You can now do it in Object Designer on the "Company" attribute by defining the default value within the property. You can also define the primary "Primary Group" in the same way.