Your analysis of your email campaign should yield big dividends if you moderate your marketing message in light of what you have learned.
You might also want to think about what other subscriber actions, if any, might you include in your analysis.
For example, who is contacting customer service, and what are the kinds of questions or comments they are communicating? Are they having trouble completing their registration. Are there technical issues with your unsub feature?
Note that some of these actions (purchases, visiting your website) take place on your website, rather than directly in the email, but the email is the triggering event.
We have been sending out marketing emails every day for the past two years, selling health information, and not a single email has failed to generate revenue. It may be the difference between 1 order and 150, but at least there are sales, and we have learned from every mailing.
Split testing, giving different offers to the ones who buy versus the ones who don’t, making great offers to your Hotlines (your newest registrants, when they are most keen and excited about your site) to get them right into your sales cycle, these are the kinds of strategies that can help make your email marketing a success.