The situation:
Self-hosted GrowthBook cannot send email: "Error sending email: Error: :error:1408F10B:SSL routines:ssl3_get_record:wrong version number". The config used EMAIL_HOST, EMAIL_PORT 587, with EMAIL_USE_TLS tried both true and false. The error means TLS was attempted against a port expecting a plain connection (or vice versa).
What actually fixes it (verified in the thread):
The maintainer fixed a bug with port 587 and simplified the SMTP settings: you no longer need to specify EMAIL_USE_TLS at all. Port 465 always uses TLS; every other port starts plain and upgrades via STARTTLS if the server supports it. Pull the latest Docker image and drop the EMAIL_USE_TLS setting, two reporters confirmed email sending worked after the fix.