Found this gem on Intercom's blog:
What everyone ought to know about subject lines
In most things in life, roughly 80% of effects come from only 20% of the causes.
80% of Warren Buffet’s wealth comes from just ten investments. 80% of software problems are caused by 20% of bugs.
It’s called the Pareto Principle, and it means that a small number of things will have a disproportionate impact. If you’re communicating with customers on a regular basis (if you’re not, read this) 80% of your success will come from those important-but-overlooked tasks – a clear focus, a personal greeting and, of course, a compelling subject line.
A quick scan of your inbox should remind you that startups tend to criminally underestimate the power of subject lines. If yours is anything like mine you’ll see that "Just checking in", "Just a thought" and the mildly terrifying "I need to talk to you" are the status quo. With email like this, it really doesn’t really matter how good your product is, or even what’s inside the email. Your recipients will never get that far.
In the same way the best editors can spend half their time on an article just writing the headline (a blog post for another day), a significant amount of time should go into a subject line that drives opens, clicks and replies.
Ok, you get it, subject lines are important, but what ones perform best? Let’s find out.
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