Thanks for the feature suggestion! While this isn't an AI solution, Thunderbird's existing Template feature serves as partial solution. Maybe that can reduce the repetitive manual replying you're currently doing?
I think a feature like this should be considered to keep offline clients like Thunderbird competitive with Web Apps like GMAIL and Microsoft 365 (they will all offer a similar functionality).
Would be nice if such AI plugin/add-on could choose from an existing set of templates and use them as an initial reply to a customer automatically without me having to that.