I'd say the easiest way would be to simply not allow postfix to reach
the rest of the internet. I.e. block port 25.
The messages will go into the queue and be stuck there for whatever
amount of time has been configured before they "time out".
They'll still be on disk in one of the postfix working folders.
:/var/spool/postfix$ ls
active corrupt deferred etc hold lib pid public trace
bounce defer dev flush incoming maildrop private saved usr
and you will be able to examine them.