[Exim] Postfix <> Exim: strange behaviour

Dear subscribers,

I've observed some strange behaviour over the last week, from two machines
running Postfix, which were delivering mail to my machine (then) running
Exim 3.16 on FreeBSD.

I noticed 'empty' messages being delivered to my mailbox, containing a
Return-path, Envelope-to, Delivery-date, Received, Msg-ID, From: and Date,
but nothing else; no body or subject.

It turned out that my /var partition had filled up and was at 102%
(stupid, but true). During that period, I only received messages from
machines running Postfix, but they were 'empty' as described. The Exim
'mainlog' just mentions the delivery of these messages, ans I see nothing
strange about them.

The Postfix maillog says this:

delay=7, status=deferred (host
my.exim-running.host [12.23.34.45] said: 452 space shortage, please try
later)

To make sure this was not Exim 3.16, I upgraded to 3.33 and tested this
agein. I'm seeing the same behaviour. Does anyone have an idea why these
messages are partially delivered, while the sending MTA does note that
there's not enough space, and it therefore defers the delivery? Is this
correct behaviour or not?

Kind regards,

--
Wilco Noordermeer
wilco@???