Email: Taking Email's Measure
Monitoring inbox delivery and immediately reacting to any issues is mission-critical, because everything else rests on you getting this one right. A 20 percent drop in inbox delivery can easily equal a 20 percent drop in revenue from that campaign. (This supposes everything is equal, meaning all names on your list have the same activity level and responsiveness.)
Opens and Clicks, Only Time Will Tell
You also need to look at your email analytics over time. That's what the ISPs do when they evaluate you as a sender. But, more importantly, your overall monthly metrics will tell you about the health of your list and your ability to sustain success.
Open rates for specific campaigns only tell you that a particular subject line was engaging, which is not very actionable information. Further, while outrageously successful subject lines increase your open rates, they usually do not result in outrageous sales increases, and responses to them typically decline rapidly over time. Descriptive, not-overly hyped subject lines that match the offer in the email copy are therefore the best approach. You don't want your subscribers to feel disappointed when they open the email, as that can prevent the sale.
Gaining an understanding of how many subscribers have opened one or more emails in a given month or quarter tells you a lot about your relevance and subscriber engagement. In essence, the goal is to have as many subscribers as possible engaged enough over time to open some of your emails.
With this information, you can work to maximize revenues or responses from your most engaged subscribers, re-engage inactive subscribers, or, if that is not successful, reduce the cadence for that segment.
If worse comes to worst, you should suppress them so they don't damage your mailing reputation with the ISPs. Sometimes a series of messages, clearly stating in the subject line that you will stop sending them emails if they don't open, can determine if they are completely disengaged or if they can be saved.