Email marketing campaigns can only be effective if done at the right time with right kinds of strategies. One of the vital tip is the "unsubscriber“
What is unsubscriber? As the pass of time,change of age, job, interest and so on. Some customers may not want to receive your emails at all? So they will want to unsubscribe. So you should add a button of "unsubscribe" in each content. And thanks their actions and ask for what they want to know more from your newsletters so that you can dig in another field of potential customers.
In another story is that, based on the development of company or the change of products or service, some email lists may be no longer meet your needs or some email lists you do not want to send emails temporarily.
I know you wont like this because no marketer wants any deletion from his list. But as a code of ethic, as a standard of respecting your customer you should give an option to unsubscribe from your newsletter. And respect his decision. Never send him a mail again.
Where could you find a software with such a function? iKode newsletter server, I think highly of it and recommend it here.