from http://bugs.musicbrainz.org/ticket/1238

We need methods to help ensure that stored moderator email addresses are valid:

Once a user's email confirm date is old enough (say, 6 months), send the user an email with a "reconfirm" URL. If the email is not confirmed within 4 weeks, remove the user's email address, and revoke automod (if they are automod).

Also when sending mail to a user (e.g. subscription, or mod note) if the SMTP RCPT command gives a permanent error code, treat the email address as immediately expired (blank, de-automod etc).

I'm not sure about the former, but the latter sounds useful.