One trap I see companies fall into when looking at opt-in and permission is they seem to think that permission is a blanket thing. They believe permission can be bought and sold by the companies that collected email addresses.

Legally, they may be correct. But in practice senders cannot decide that they can sell the permission of their recipients to another entity. You cannot buy an existing business relationship.

A study of success metrics for marketing e-mails sent through MailChimp’s distribution service showed that Gmail users were most likely to open and click on e-mails.

Excellent article by Bob Frady. If you're an email service provider, please read this and then put your developers to work! ;-)