We all know that you can set a forwarding address for a specific person in Domino. However, if you do it from the Domino Administrator (Person Document Properties/Mail tab), the person's mail file does not keep a copy of the forwarded messages. There are alternatives where you can create an agent to solve this dilemma, but it is up to the end user to turn the agent on/off.
Another alternative that does not need user intervention would be to create a person document, call it "Prime", that points to the same mail file as the user who needs to forward his/her emails (called "Initial"), and another person document, call it "Forward". Create a group, call it "Forward Group", which contains both "Prime" and "Forward". Now set "Initial" to forward messages to "Forward Group". This will forward the message to all the members in the group, in this case "Prime" and "Forward". Set "Forward" to forward messages to the intended forwarding address, for instance firstname.lastname@example.org. Since "Prime" will deliver messages to the same mail file as "Initial", the mail file will contain a copy of the messages forwarded to the internet address outside the organization.