[MM3-users] Captcha or similar for new signups?

Hi Folks, Does anyone have a recipe for adding captcha or similar to stop bot accounts signing up with Postorius? We're seeing a heap of fake signups resulting in confirmation emails going to people who haven't signed up, with user names like 'CAOXpYhPFPBFr' -- I'd like to make it just a little harder for bots to sign up.

Too many confirmation emails going to non-existent email addresses reduces our mail reputation, and increases the likelihood that legit email is flagged as spam.

Peter C