When you send out your campaign, you think that you send it, promote your business, and then your clients will contact you to buy from you. But in reality, it does not work that way, you could actually interact with your clients, and have them click on a button to reply to you, no matter how far they are from you.
I strongly recommend that you stay in touch with your clients, ask them to contact your sales representatives or even you directly, if they have any questions or inquiries. This way they will know that they can count on your company, if they need assistance, and they will be delighted to know that your company is there to assist them at any time.
Send them surveys, and this way you can interact with them, and know what your clients think, not only about your products but also in regards to your newsletters. You can create a section on your newsletter, where you can put this results, and that way keep them updated.