Easy, set up a campaign (or rather a two-part campaign, as I'll explain) that triggers on web page visits, waits a reasonable period of time, checks that they did not check out a cart already, and then sends them the email. These work best as two-part campaigns, so that you trigger the first campaign with the visit and a wait step, then hand it off to another requestable campaign that has a filter set up on it to cull out the ones that checked out a cart during that wait step before sending an email to the ones that didn't.