Subject header not detected after folded header

[Ben Wylie]

Am running SpamAssassin 3.1.2 on Windows 2003 server.

This is an extract from the headers of an incoming email.
This triggered the "MISSING_SUBJECT Missing Subject: header" rule.
Why did this not detect the subject header?

X-MimeOLE: Produced By Microsoft Exchange V6.5
Content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: multipart/alternative;
        boundary="----_=_NextPart_001_01C6B494.1CB001D7"
Subject:
Date: Mon, 31 Jul 2006 12:26:05 +0100

Thanks
Ben