Well, it's reasonable to shut down a site's email if it's possibly a spammer (which tends to generate a lot of bounces). It would be nicer if it could be applied to an individual email address rather than the entire domain, but that's what it is (for now, anyway). LP has to balance the inconvenience of site emails being temporarily shut down against the death penalty of having the entire server being blacklisted as a spammer.
Why would a site on a shared server have "a significant number of email addresses", with uncontrolled users who might be generating spam? That sounds dangerously close to a violation of the ToS. I suppose this might happen by accident with, say, a store or forum trying to email a newsletter to all its customers or members, and some have bogus email addresses. That's not spam, but the mail system can't tell why bounces are happening, only that a significant number in a short time is a potential sign of spamming.