You can do this. You'll just need to define different classes of traffic (using access-list matches) and then inspect ESMTP on certain classes. For example:
access-list traffic-with-TLS permit ip 192.168.1.0 255.255.255.0 any eq 25
access-list traffic-without-TLS deny ip 192.168.1.0 255.255.255.0 any
access-list traffic-without-TLS permit ip any any
class-map inspection_without_smtp
match access-list traffic-with-TLS
class-map inspection_default
match default-inspection-traffic
match access-list traffic-without-TLS
policy-map global_policy
class inspection_without_smtp
inspect dns maximum-length 512
inspect ftp
inspect h323 h225
inspect h323 ras
inspect rsh
inspect sqlnet
inspect skinny
class inspection_default
inspect dns maximum-length 512
inspect ftp
inspect h323 h225
inspect h323 ras
inspect rsh
inspect esmtp
inspect sqlnet
service-policy global_policy global