The biggest mistake I made in an email campaign was focusing too much on the message and not enough on the audience. I sent the same email to everyone, assuming it would feel relevant, but engagement dropped because the content didn't match different interests. Watching open rates, click patterns, and unsubscribe spikes quickly revealed the problem. Reviewing subject lines and send times also showed missed opportunities. Instead of guessing, I started segmenting subscribers, personalizing content, and testing small changes before larger sends.