Best AI Subject Line Optimization Tools in 2026
4 tools, each with the price we read from the vendor and the date we read it.
4 tools · checked 20 Aug 2026
4 tools in Subject Line Optimization, with monthly plans running from $49 to $180, a median entry of $180.
2 have a free tier rather than a trial (beehiiv and Klaviyo AI), and Persado publishes no price at all.
They do not meter the same way — this page mixes quotes, fixed plans and a calculator — so compare what a price buys before comparing the price.
P Persado Enterprise marketing language, tested for response pricingfree tierapi 
beehiiv Newsletter platform with AI writing and growth tools pricingfree tierapi 
Klaviyo AI Lifecycle automation with AI segmentation for ecommerce pricingfree tierapi 
Seventh Sense Per-person send-time optimisation for HubSpot and Marketo pricingfree tierapi
beehiiv Newsletter platform with AI writing and growth tools pricingfree tierapi Klaviyo AI Lifecycle automation with AI segmentation for ecommerce pricingfree tierapi Seventh Sense Per-person send-time optimisation for HubSpot and Marketo pricingfree tierapi prices read from vendor pricing pages · 20 Aug 2026 · how we verify