Which action is a valid fallback when a client certificate is unavailable during SMTP authentication...

SMTP AUTH is a valid fallback action when a client certificate is unavailable during SMTP authentication on Cisco ESA. SMTP AUTH is a method of authenticating SMTP clients using username and password credentials, which can be verified by an LDAP server or a local database on Cisco ESA.

References: User Guide for AsyncOS 15.0 for Cisco Secure Email Gateway, page 5-10.