What kind of offers do you usually send to your users? Is it really workable? How could you turns them into the benefit of your business? Actually I often found huge offers in my junk mail and most of the time same mails have been repeated two or three times. And it looks quite irritating too. I never indulge myself to see them. Although, most of the time I have to push the "unsubscribe" button (where as I never ever subscribe for this kind of mail.) So my question is that, don't you think that, it is not the right way of marketing....
One of my lists is for Kindle authors. I only send them offers that will help them in their publishing on Kindle. So, what sort of "users" do you have? What type of offers would they be interested in?
If you keep sending offer after offer to your subscribers ofcourse they would get irritated I would. The key is to send value. If you send value then you gain trust then in the future it will be easier to make sales.
Make your offers infrequent and highly targeted to your audience. If you have a large enough audience, you should be testing multiple targeted offers to see which of them convert best. I am never offended by an offer for a product or service that is of interest to me. All others hurt themselves by contacting me.