I actually found it odd when you guys sent your first 15 emails with just a number in the subject line. I remember assuming you were probably testing something.
Anyway, maybe in the future you can test something like Persado https://persado.com/ (Godlman Sachs invested in its latest round in Feb 2016).
Here’s an image of its efficiency:
Found it thanks to Nanalyze’s article:
May assist in the next email adventure.
Take care folks