To your question, yes, you must configure access-list to lower interface. At the moment, you use access-list on an interface, you must specify all rules to permit/deny any traffic passing through this interface.
Unfortunately, ACL affects default behavior. Then, you use it or not. But, since your access-list only apply to SMTP servers between inside and DMZ, you can use "NAT 0 access-list id" between those servers. The "NAT 0 access-list id" is apply to inside interface and leaves incoming or outgoing traffics that match the access-list specified. That way, i suppose you can simply remove your ACL 110 applied to int3, to keep default behavior (browsing), and permit traffics between your SMTP servers.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...