If you’re using email marketing software now or plan to in the future, you’ll find that reputable companies will insist that you use opt-in email lists. You might be saying, “I’ll just use a non-reputable email marketing vendor.” Alas, ESPs on shared IP addresses that don’t require customers to use opt-in email lists typically suffer poor deliverability. Why? One customer’s ill-gotten email address list can poison the deliver ability of the other customers on that shared IP address. You’re going to want to hitch your wagon to the light side of the email marketing force if you want your emails to actually get into inboxes.