Spam with attachments and UNPARSEABLE_RELAY

[Geoff Soper]

For a few weeks I've been suffering spam messages with attachments 
getting through with a suspicious score of 0.0. Upon inspection, they 
all had the following lines in the header:

X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on
        server.alphaworks.co.uk
X-Spam-Level:
X-Spam-Status: No, score=0.0 required=3.0 tests=UNPARSEABLE_RELAY
        autolearn=unavailable version=3.3.2
X-Spam-Score: 0.0
As the first step in trying to resolve this, I updated to 3.4.1 which 
results me in sometimes seeing (as expected):

X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on
        server.alphaworks.co.uk
X-Spam-Level:
X-Spam-Status: No, score=0.0 required=3.0 tests=UNPARSEABLE_RELAY
        autolearn=unavailable autolearn_force=no version=3.4.1
X-Spam-Score: 0.0

but also sometimes:

X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on
        server.alphaworks.co.uk
X-Spam-Score: 0.0

i.e. missing X-Spam-Level: and X-Spam-Status:

Can anyone suggest what is going on here?

Thanks,
Geoff