Which action is a valid fallback when a client certificate is unavailable during SMTP authentication on Cisco ESA? 

Answer options:

A. LDAP Query
B. SMTP AUTH
C. SMTP TLS
D. LDAP BIND

B

Reference: https://www.cisco.com/c/en/us/td/docs/security/esa/esa12-0/user_guide/b_ESA_Admin_Guide_12_0/ b_ESA_Admin_Guide_12_0_chapter_011011.html