I could recommend a private or vps server ... If you need to send more than 100 mails an hour, that is probably a good way to go. Look at it from the other side; there are countless idiots who try to use shared webhosts to spam; then there are countless more who have vulnerable PHP scripts on their site that then get used to spam. This may not apply to /you/ specifically, but only a few of them getting through the defenses will land a good portion of dreamhost in blacklists. Also note that the more mails you send, the more CPU power will be consumed. On a shared host, that can spell disaster, especially if DH were to use outgoing spam filters, as well (which cuts down on blacklisting a lot, but at a price; each message would take upwards of 1s of raw CPU time to get out the door. That's a lot, considering you want to send > 2400 a day
Filtering incoming mail is not so easy a prospect, either. It's in no was "BASIC". Spam does not carry a flag that says "put me in junkmail, please !". A lot can be done to improve this situation, but to become truly good at this, you'd have to shell out quite a bit more money; for one thing you'd want to subscribe to some of the bigger signature-based live blacklists, possibly get mail scored by an external relay, constantly refine the filters (optimally on a user-by-user basis, since not everybody gets the same kind of spam), and have several machines DEDICATED just to SpamAssassin and such. Big machines with lots of processors and memory. (the more effective spamassassin gets, the more rules it has and the more complex those rules get. That takes major computer power). ClamAV can also reduce the spam intake a lot, but also at a processing cost (albeit that is much smaller than SA).
DH may not have the best mail service in the world, but I suspect you'll end up paying quite a bit more for a (web)host that does. It's not an easy problem to solve.