Paste a subject line. This flags seven patterns that make one read as a sales email before it is opened. It runs in your browser and nothing is sent anywhere. If you would rather not write the email at all, Leaderr's AI-written proposal emails draft it from the prospect's own domain.
It checks form, not fit. A subject line can pass all seven and still fail, because it was sent to someone who does not have the problem. Nothing about wording fixes a bad list.
The single most useful test is not automatable. Could this subject line have been sent to anyone else on your list? If yes, it is doing no work. A line that names something true about one recipient cannot be mass-sent, which is exactly why it gets opened.
Tricks are excluded deliberately. Fake reply prefixes and invented urgency do lift open rates, and they destroy the conversation that follows. Someone who opens because they were misled marks it as spam, and that cost lands on your whole sending domain.