Every channel of sales can be the best and the worst. And only email marketing has a lot of options. Spam email marketing, email marketing with registered users, email marketing with users who sign up for some free e book or some free product, email marketing with users who bought you product, reselling, up selling....
Email marketing with existing users take a lot awards for the best channel of selling over 20 years. I using and sms marketing too, it is great option for users who allow and confirm their phone numbers for info .