While both direct mail and e-mail marketing are effective sales tools, they are not interchangeable, and should be used differently to maximize results.
E-mail is most effective at communicating with current clients. Since you already have a relationship you can use this to notify them of special offers and move product.
To convert a prospect to a customer you have to build trust which is more difficult with e-mail since many people are conditioned to view e-mails from unknown companies as spam.
Direct mail marketing is more effective at converting prospects to customers. With direct mail, you have a better opportunity to tell your story and engage the prospect.
Know your prospect. If you are targeting middle aged professionals, this demographic is probably most open to direct mail solicitation. Younger prospects are probably best engaged through social media.
For more tips check out the complete aritcle from Chief Marketer.