I'm a developer at Clarity and was involved in the development of this feature.

This specific feature was developed in house. We keep event logs around for a log of things and this feature is built on top of that. When you visit a certain page showing interest in an expert, we'll schedule a job to send this email. A few hours later, when the job runs, it verifies a few things against other logs and we send the email if it's a good fit.

These days I would take a good look at Intercom (https://www.intercom.io/) to do this. They recently came out with event tracking (http://insideintercom.io/power-of-behavior-driven-messaging) making this kind of feature a lot easier to build.