Don't send users more emails if their address doesn't work

[ozamosi]

Over time, old users' email addresses might start to bounce. Implement way to give up reaching users when their email keep bouncing.

Thoughts:

• We should allow the odd bounce before giving up completely.
• We shouldn't delete accounts of users who are still part of active organisations, and/or log in regularly. Perhaps a "invalid email" field that stops us sending emails and lets us implement a "could you please update your email" popup on login would be a better solution.

@mnowster @morancj