Two particularly important groups for our purposes are customers with a one-time purchase and customers who have purchased multiple times. A customer is sometimes not considered to be loyal or repeat customer until they have purchased two to five times, in which case the single purchase segment is more akin to a warm prospect than a loyal customer.

This seems pretty straightforward. It’s about matching email metrics to your marketing objectives (remember them?) If it’s all gone well, your email conversion rates and financial metrics should track closely with your marketing objectives and customer journey. But, if it’s not doing that, don’t throw your hands up in despair! Look into the data, see what’s happened, work out why, and extrapolate some lessons for future campaigns.
A trigger can also be a passive customer behavior, such as not opening your last few emails, not logging in to use your service for a while, or downloading a product without downloading the tutorial or an important related resource. These types of drip campaigns can help re-engage customers who were moving through the customer lifecycle but have somehow gotten “stuck.”
You can assess the interests of individual list members by monitoring which pages they land on, and where they go from there. The process is explained by this example from Airbnb. They use click behaviour to ascertain the destination preferences and travel styles of customers and base their follow-up emails on those preference profiles. It's intensely specific, but it gets results!

Alternatively, at least, an eye-catching design. It’s worth letting a proper designer have a go at prettifying your emails. Customers are often opening emails on the go, so you need to work hard to grab their attention. Making your emails nice to look at is a brilliant way to hold their eye. Netflix put time and effort into making their emails stand out, personally and visually – and it pays off for them!