Is there a way to have a script to mark Authenticated mail connections and allow that via IP Firewall mangle and Filter rules, ie drop mail that's unauthenticated on port 25 but allow authenticated mail on port 25
It would likely depend on if the port 25 communication was also encrypted (SSL/TLS) or not.
If it was unencrypted then one could create a L7 filter or content rule to detect the presence of the authentication commands in the connection however if this was encrypted there is no way to perceive what is occurring with RouterOS