AS2 inbound error

We have a TP that we are unable to receive AS2 communications. We are running IS 6.1 SP2. We have exchanged certs and we can send them an AS2, but when they attempt to send something to us using Cleo/Lexicom, they get:

2007/04/23 13:19:44 Exception: " Receiver’s connection prematurely closed!
at com.cleo.lexicom.beans.as2bean.httpSender.sendBlock(
at com.cleo.lexicom.beans.as2bean.SMimeClient.sendCompressedSignedAndEncryptedMessage(
at com.cleo.lexicom.beans.as2bean.SMimeClient.sendMessage(

We have nothing in the error log or server log (that I can tell) and no indication that wm.EDIINT:receive has been invoked.
We have a reverse invoke server, but no error log entries there, either.
Any suggestions?

Are they being blocked at the firewall? Also, can you verify the URL they are using and that it works for you from a machine outside your network?