Add documentation on managing email lists

We'd like the manual to cover how recipients are filtered from getting an email (if they've opted out, they won't receive any bulk or one-off emails; what happens to folks marked "do not email" — not automatically excluded, but can be through adv search; the different bounce categories and how to deal with them — some mean that the inbox was just full, for example, and some mean that the email is invalid, meaning that you should remove those folks from your email list so as not to overcount your recipients, etc.).

I'm attaching PTP's documentation on the subject that you can borrow or adapt as you see fit.