The forum currently allows new users to send PM spam. This is especially bad because it sends outgoing e-mail from the server which results in bitcointalk.org being classified as an e-mail spammer.
Even if new users were disallowed from sending PM's, or at least PM's that result in e-mail, that still would not protect the forum from being listed as an e-mail spammer. Compromised old accounts could send spam PM and get the server in trouble.
One solution that would substantially help this is spam filtering of outgoing PM e-mail. Perhaps the outgoing message can be piped through spamassassin. Spam score and particular rules like URIBL's can be used to flag an account and possibly stop outgoing mail that could get the server in trouble.