Considering the error message indicates the address that failed was
irv2news@irv2.com, it appears that the member was attempting to reply to the 'notification' of the PM rather than replying to the PM itself. Such a reply normally would be accepted by the iRV2 server as opposed to being posted as a PM reply by the Infopop server, but as we've discussed in several other threads located in this forum and various other forums, the iRV2 server is currently having some gremlins exorcised from it which started early yesterday morning.
Even if the server were working, members shouldn't reply in that manner because you'd not ever see the reply unless you were privy to iRV2 email.