[Exim] Missing headers bypassing spamassassin

Folks,

we're running exim 4.20 on our mailhubs with rbl, exiscan and
spamassassin. This all works very well but I've received 3 messages over
the last week that haven't been tagged with a spam score by
spamassassin. On further investigation these messages had some of the
expected rfc822 headers missing. One was missing Date, Subject, To and
message-ID the others just Date and Message-ID. Is there any reason why
exim hasn't at least added Date and Message-IDs and is the fact that
some of these headers are missing causing the messages to by-pass spamd?

Thanks,

Clive McDowell

Information Services
The Queen's University of Belfast