Hi
It is better if you place the Mail Server in the DMZ zone and accordingly you have to restructure the rule base.
Since you only need to scan the internal mail traffic, only one service policy is required on the inside interface, with an access-list that matches traffics to be scanned.
access-list local_mail permit tcp eq 25
access-list local_mail permit tcp < internal_network> eq 110
ASA5510(config)# class-map mail-traffic
ASA5510(config-cmap)# match access-list local_mail
ASA5510(config)# policy-map mail-pol
ASA5510(config-pmap)# class mail-traffic
ASA5510(config-pmap-c)# set connection per-client-max
ASA5510(config-pmap-c)# csc [fail-close | fail-open]
ASA5510(config-pmap-c)# service-policy mail-pol interface inside
Hope this will serve your purpose.