Why it works
The brain experiences a literal itch when it notices a gap between what it knows and what it wants to know. Introducing a mystery forces the reader to open your email just to satisfy that mental hunger.
Example
I found a blind spot in your current marketing strategy that is currently costing you about twenty leads per week.
An illustration of the mechanism, not a template. Copied verbatim it stops being persuasion and starts being the pattern everyone else is already sending.
Is your sequence pulling this one?
Paste it in and find out which levers are working, which are missing, and which single zero is capping the whole message.