It needs to be able to have devices connect to it from outside? Devices on the outside would initiate the connection? That is the only reason it would need a firewall rule. Unless you had specific outbound rules.
Most of these types of machines need to have an "account" with which to recharge or determine how much postage you have available on your prepaid account AND/OR a way to report what kind of postage it has stamped on your correspondence.
We have the same thing, if you don't let it's traffic out then you will always need to take the machine somewhere else to "recharge" the postage dollar amount it is allowed to print.
The owners manual and your contract should have stated what the access was used for.
What would Clark Kent do to someone who stole his identity?