I get the concept, but naming it "Sendability" might confuse people with deliverability scoring. What you're really measuring is opener quality - how compelling the hook is. Something like "Opener Strength" or "Reason-to-Reach Score" would be clearer.
For a solid opener, I look for three signals: a recent trigger (hiring, product launch, complaint post), a clear reason why it matters to them, and a natural bridge to what we do. Not just "saw your LinkedIn post," but "saw you're expanding the sales team - here's how we helped similar groups shorten ramp time."
Weak signals to avoid: generic job titles, stale funding news, or scraped "congrats on growth" - everyone sees through that. Strong signals: recent tool switches, public pain points, new campaigns, pricing pressures, or anything they've said in the last 30-90 days.
Separating the score into two parts would make it far more useful: "hook strength" for the opener quality, and "send risk" for deliverability signals (bounce rates, domain reputation, list hygiene). We work on SMTPProvider, and from our side a perfect opener still lands in spam if the list is messy. If you can feed delivery data back into the score later, the feature becomes genuinely actionable - not just AI judging copy.