I'll give you a warning about the headline testing feature. If you're the sort of person that releases a paywalled version of an article and then sometime later that article gets sent to free subscribers, what you're doing is just sending two emails to the same person twice with a time in between. Problem is, when you do that with the title test, Gmail notices that they're two very similar emails but slightly different.

Ordinarily, if they're identical, you haven't touched it, it will be threaded, and Gmail will just put it into a thread, and they will still get the second email delivered with the paywalled content. But if you do a title test, Gmail will register that they're slightly different, and what it will do is it will fail to deliver the email to everyone who got a different title. I found this out when I was tweaking my headers above the paywalled prior to the free version coming out, and it basically completely shafted me for most of the start of the year because people were just not getting the free unlocks of the email, and it was pissing off all my free subscribers. This title testing feature needs to be taken with a pinch of salt.