Best metric to measure email's success?

I use different metrics in different situations:

- Deliverability for new leads or lists from events

- Open rate for subject line testing

- Unsubscribe rate is especially good for tracking content to customer contacts (since we have a relationship, an unsubscribe means more to me in that context than prospects).

Generally click to open rate is my go-to metric since it also pairs nicely with Marketo success steps in a lot of cases and helps me align how things like subject line (what they expected the email to be about) work. However, I've done A/B tests on subject line and had virtually identical open rates but an insane difference in click to open rate. This indicated to me that while both of my subject lines resonated, one actually provided a better descriptor of the content so people clicked through. The other subject line must not have matched the content the audience expected.

In short, I don't think you can drill down to just one metric, you need to look at the whole picture. You choose the metric that illustrates what you want to say about the success of an email.

I report on all of these things listed because I think they each tell you something different, and you need all of them to get a more holistic picture of your email's performance.

• Deliverability - Delivered / Sent - obviously this gives you an idea of how your deliverability is performing for this list of people. Sometimes we see a dip when we are sending to a purchased list for the first time. I also noticed our DKIM had never been set up after investigating a low deliverability score.
• Open rate - Opened / Delivered - this tells me how much subject line and/or email from name performed. When I see a lower than expected open rate, I usually tweak the subject line for future sends (like in an Engagement program)
• Click-through rate - Clicked / Delivered - I don't look at this as much as I look at clicked/opened, because I think clicked/delivered is unfairly dinged for a poor subject line. If the people who did take the time/find value enough to actually open your email ultimately clicked, I find that more valuable than those who never opened and never clicked.
• Clicked / Open - see above, I use this most
• Lowest Unsubscribe rate - we look at this too, and if we notice a particularly high unsubscribe rate, we will investigate if we sent to a list that was not expecting to hear from us, if our email looked spammy, etc.
• We do not measure a single email's success, we look at the parent program's success - of course we do take all of this into consideration under the umbrella of the program's performance overall. If a series of emails all performed poorly, did we have the wrong message for our audience? Were they not warmed up enough before receiving this communication? Or are they just the wrong people to target?

Hope that was helpful!