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