Since the attachment is encrypted it cannot be intelligently scanned.
The only option I can see in the ironport C-Series is to change the Incoming Mail Policy, Antivirus, Encrypted and/or Unscannable Messages Action to Deliver. If you don't want to do this for everyone, create a separate policy group for this sender where the above is done.
I'm not too familiar with Ironport Encryption Appliance. Some systems of that type allow managing keys for specific external email addresses, though they are generally geared for outbound email flow they do handle replies.
