[barryrandall]

2 rules I follow: Don't get flagged as spam. Don't send messages to unvalidated email addresses.

Spam- Few things move faster than an annoyed user's mouse going for the "Report Spam" button. It should be 1 click, with links at the top and bottom of the message. Do it in real-time, not in "24-72 hours." No logging in to change communication preferences. Don't bury it in fine print or hide it with CSS.

Invalid Addresses- Some email providers penalize (by slowing delivery) the originating IP address every time that IP attempts to email an invalid address. If your mailing list software then retries delivery to a bad address, the penalty increases (exponentially with some providers). This can create denial/degradation of service vulnerability for your outbound mail.