When your subscribers aren't in the mood to buy, your e-newsletter should still be interesting enough to garner a few moments of their attention.
Communication is the bottom line; buyer conversion is the icing on the cake. When your readers aren't in purchasing mode, relevant and interesting content may be all you have to fall back on. This is what builds the trust and appeal that propels those future sales.
If your readers are bored by your newsletter, either because it's saturated with promotions or they find it difficult to apply to their immediate needs and interests, you can safely predict that they probably won't open it again. Furthermore, they might just give their business to a competitor that's managed to keep their attention.
A tried and true method of keeping subscriber interest is list segmentation. Dividing your recipients into categories based on their proclaimed interests and how they have interacted with your newsletter/website is the best way to know how to formulate the range of topics, media and promotions you earnestly send their way.
Send surveys to get reader feedback and study your reports carefully so you'll know how to hone your content accordingly. If a particular subject line made open rates soar, you know how to focus future content.
Your readers will appreciate the fact that you're acknowledging their interests and not just trying to make another sell.