I've never witnessed it personally here at DreamHost, but there was some issue recently on one of the mail servers with spam. Unfortunately with shared hosts this kinda thing happens - it's inescapable. Any shared host who attest to sending absolutely 0% spam 100% of the time would be telling furfies. Having said that, DreamHost do appear to have a pretty good record for getting any trouble sorted out very quickly. They immediately banhammer any account that indulges in spam-like activity and chase up potential blacklisting concerns post-haste.
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