I have a courier installation running courier 0.60.0 which delivers _most_ email from the system and for remote clients using it for relaying with no problem. Occasionally, however, something like the following happens:

Dec 5 09:02:07 shakti courieresmtp:
id=000000000021DBA1.000000004939426F.00003979,from=<user@redacted>,addr=<someone@redacted>:
No such file or directory Dec 5 09:02:07 shakti courieresmtp:
id=000000000021DBA1.000000004939426F.00003979,from=<user@redacted>,addr=<someone@recacted>,status:
deferred

This seems to be associated with particular (remote) addresses, since the emails are queued for retry and fail repeatedly. Other (most) addresses have no such problems and the emails are delivered normally. If I telnet to port 25 on the server and send an email manually I don't see a problem in the SMTP dialog (I get a "250 Ok" after the data segment followed by the message ID), but if I check the mail log I see the same "No such file or directory" message. Is this something that's coming from the remote end without a proper error code, or something that's being generated by courier?

What gives here? Any clues?