Hi Marcos
I don't think it's related to the firewall because there are able to reach the AS2 endpoint, plus your testing from Mendelson gives the same error.
From the AS2 manual, below are the main reasons for such an error.
My suggestion would be:
i) Check that there are no cache issues. Try to refresh the cache and check that the sender agreement and sender comm channel is reflected in the cache
ii) Try to create a new scenario with different set of sender and receiver party with different AS2IDs. Test this new scenario using the Mendelson tool. If it works, then we can narrow down the issue to the specific configuration rather than your system as a whole. For the test, best to just skip encryption and signing to simplify the scenario.
iii) Check also the Message Monitor in the Seeburger Workbench. See if there are any errors there that might provide more details.
Rgds
Eng Swee