Email bounces from dead accounts become new issues

We're having a rough time keeping up with user changes. When a user leaves and corporate disables their email, there is no way to tell JIRA to stop sending them email, and because JIRA's email handler isn't smart enough to ignore mailer-daemon bounce messages, we've ended up with lots of erroneous issues and comments.

So, I see a couple possible solutions:
1. Make JIRA email handlers smarter so they learn to ignore bounce messages
2. Make email address an optional field on users so an admin can remove it when they are disabled.
3. Provide an official "deactivate user" function (removing from jira-users is clearly insufficient). This has been suggested earlier.

Support suggested doing a bulk change so nothing was assigned to those users but that doesn't solve the whole thing. If they're the reporter, they'd still get emails. Or what if they're watching things?

Changing their email to a known good one (like my own) was tried, but that prevents that user from emailing in issues because JIRA doesn't know which user my email goes with.

Maybe someone can suggest another solution I don't see.