New to Spamassassin, some questions

Okay, I'm brand new to this hosting service, but not new to hosting or administration. DH seems to handle things a bit differently than the hosts with whom I have been previously acquainted, so I'm a bit confused as to how some things work. I checked through the Wiki, and a lot of posts on here, but have yet to find distinct answers to a handful of questions. I hope you folks can help me out.

1. DH seems to consider "mailboxes" and "users" seperately. Can users receive email without specifically setting up "mailboxes" for them?
2. I've never used SpamAssassin before, and am pretty confused by it. I'm familiar with several server-side spam filtration programs (especially TMDA: http://tmda.net), which use customizable filters and white/black lists, all of which are directly available from a web interface, and easily applied to all the mailboxes on the domain. Does SpamAssassin offer such features?
3. Would installation of features such as TMDA and other filtration software be possible on a DH domain, and would such software be applicable to the "mailboxes" created through the DH control panel?
4. Are blacklists available, and if so, are there methods for bulk-editing them?

Thanks all I can think of now. Thanks to any who can offer advice and assistance.

- Nathanael

Can any of you help me with this?

Can't help you with your two following questions, but when you first set up a user account you are prompted for the email address that it will use, which can be entirely different in name from the user name. As opposed to a "pure" email account, in which the mailboxes are hosted on DH's mailservers, the user account's email will be hosted in that user's account space / webserver, which is necessary in order to have complete control over the email, most notably setting up your own SpamAssassin install. Note, too, that the user name you pick must be unique across several ("a cluster") of DH's servers, tech tells me, but since, as I said, you can pick the email address separately, this needn't be a concern.

This, at least, is what I've gleaned so far in my ongoing effort to get a single SA install to work for multiple users.