Acquisition: only permission based, and by that I mean these customers explicitly requested you contact them. And only when relevant, timely and targeted. The only other time is when it is a referral, and that must be a one time mailing that explicitly states where the lead came from. Otherwise, you’re spamming.
Retention: Excellent. Again, timely, targeted and relevant. But don’t overdo it. Because you don’t want to lose them as customers via your “retention” efforts if they’re more of a sales pitch. Too many marketers confuse retention with “Let’s sell them more stuff!”
Email’s Biggest Positive: One thing many marketers miss: selling in ancillary product to existing satisfied customers, which may be it’s biggest benefit. But don’t overdo it. Many companies simply send too many emails, and don’t give customers control on how many they’d like to get… daily? weekly? monthly? If you don’t have those controls in place, and simply have an on/off switch, you risk losing a good customer.
Just because it’s cheap doesn’t mean it’s good. Don’t rely on email or Twitter for all the heavy lifting. Depending on the product and audience, direct mail, social marketing (such as Facebook fan pages for consumer marketing and LinkedIn networking for B2B), business blogs, newsletters and personal contact (in-person or telephone in B2B) work well. Even things like Twitter if used smartly.