Here is my plan for forwarding. Let me know if this looks correct. First off, I deleted the list off of the old server, so I am assuming that removed the alias as well.
-Create new alias file in /var/lib/mailman/data/ called migratedaliases (this directory is where the aliases file is currently located, but under the aliases file). -Edit /etc/postfix/main.cf and add the path to migratedaliases to alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases. After editing, it will look like this: alias_maps = hash:/etc/aliases, hash:/var/lib/mailman/data/aliases, hash:/var/lib/mailman/data/migratedaliases. -Run newaliases command to update the aliases db. -Then add the alias for the the migratedaliases file the way you posted above.
Does that make sense?