What's On Display
Out of respect for my direct marketing email list I never include more than three items of interest in any one message. My goal is to focus on just one thing. If I find the item useful and exciting I don't want to detract attention from it.
Ever walk into a department store where the display racks have taken over? It's hard enough to navigate through the aisles and find what you're looking for. Between the stacks of products and the gaudy signs a person can be driven to distraction.
Nothing in Common
The only reason I ever consider the second or third item of interest is when they compliment one another. For instance if I'm marketing a book on 'Caring for House Plants' and I know about an available online course or a video that explains the same topic, I'll add those links as a matter of convenience.
Anyone on a direct marketing email list has already opted in for a specific reason. The people who have an interest in indoor greenery aren't necessarily interested in organic farming. If I don't appreciate the warehouse attitude of, 'Here's everything in one place, go and find it,' why would I want to open an email that does the same?
They Can Always Opt Out
Permission marketing is the principle behind building a direct marketing email list. Given the choice, shoppers will likely leave a cluttered display-crazy store to find a more pleasant experience. If we hit our list with enough junk they're not interested in, they'll be gone.
Direct marketing through regular mail or email is effective because the message is usually short, to the point and of interest to anyone on the list. People stay because they buy items of interest to them. They won't wander around or read a bunch of ad copy for very long before leaving for good.