How can you send a second pass email to customers that didn't open it the first time around?

In order to send a second pass email, you can first copy your existing message after you finish sending it out:

Then pop into the copied message and setup a segment on it using Opened Messages isn't one of:

And select the original message in the list of messages.

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Pro Tip: While you're in there, give the message at least a slightly different subject line for good measure.