Leave the customer with the impression that their request was promptly and professionally dealt with, and they can say no wrong about you. Fail to remove them for two to three mailings and you'll rile them up. There are nasty people out there just waiting to make your life a misery. It's critical that you manage removals effectively and promptly. Time your messages to coincide with real world events.
Most people read their mailings at their office desk.
The strongest response to your content will likely be a mid-week mailing. Be sure to send this out late the night before your intended time of delivery. Large mailing list can take hours to deliver. If you have a time-sensitive message that contains information about a weekend sale, you'll want to get this out the door by Thursday evening. There are folks who depend on the subject line as an indicator of the content.