The purpose of checking the Certificate Revocation List (CRL) during SMTP authentication on a Cisco Secure Email Gateway is to check if the certificate is not revoked by the issuing Certificate Authority (CA). A revoked certificate means that it is no longer valid and should not be trusted. Reference = [User Guide for AsyncOS 12.0 for Cisco Email Security Appliances - GD (General Deployment) - Configuring SMTP Authentication [Cisco Secure Email Gateway] - Cisco]