Here's a quick fix you should do first. In your ACL permit your internal mail server to send SMTP and block everyone else. This should go at the top of your ACL.
access-list inside_acl permit tcp host mymailserver any eq smtp
access-list inside_acl deny tcp any any eq smtp log
That will prevent the email from getting out sourcing from anything other than your approved mail server. Next you can take a look at your logs and see who is getting denied. The logs are generated from the log keyword at the end of the ACL line #2.
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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...