Two Clicks

There's a nice hierarchy in email marketing.

1. Segment buyers by the Elite / Loyal / Quality / Average / Struggling / Lapsed framework.
2. Segment email subscribers by Inactive / 1 Click-Through Past Year / 2+ Click-Throughs Past Year / 1+ Email Purchase Last Year.

Overlay (1) by (2) and you have something!

Regardless how many email campaigns you send per week (or per day) clicking through two campaigns in the past year is a meaningful metric ... those are the email subscribers you focus extra effort on.