The SMTP STARTTLS upgrade performed by airflow.utils.email.send_email now validates the SMTP server's certificate against the system's trusted CA bundle by default. Previously the starttls() call was made without an SSL context, so any certificate was accepted.
Deployments that intentionally point Airflow at an SMTP server with a self-signed or otherwise non-validating certificate and need to preserve the previous behaviour must set email.ssl_context = "none" in airflow.cfg. The "default" value (now also the default when the option is unset) uses :func:`ssl.create_default_context`. Previously this option applied only to the SMTP_SSL path; it now applies to the STARTTLS path as well.