AOL has enormous trouble with spam, but they have massive resources (and deep pockets) to try to take care of it. Their spam filter (which is always on) is extremely harsh, and they are very slow at removing blocks against domains that have been spamming.
Most blacklisting I'm aware of is automatic, and triggered by rates. The block is removed manually after a successful appeal. I know that some organizations (like AOL) also do aggressive filtering, but that is only practical for receiving email, not sending it.
Only mailing lists (or high-rate users) need to be kept separate. For normal mail operation, you could have a far stricter rate limit (like 20-per-hour or something) to prevent the main system being compromised.
It isn't as simple as that, though. When an ISP blacklists the DreamHost email servers, DreamHost usually responds almost immediately (because they get a bunch of support tickets telling them they can't send emails to folks!). The problem is that the organization doing the blacklisting is sluggish in removing the block. AOL, for example, can take days to remove a block. Other ISPs make it very difficult to contact them to try and get a block lifted. Blacklisting agencies are even worse.
Basically, the whole email system is broken. We need Email 2.0 more urgently than any other new internet technology.
Simon Jessey | Keystone Websites
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