Header handling question

[NFN Smith]

One of my spamtraps is getting a lot of traffic of messages with 
Facebook and Twitter URLs.  This is content that is coming from Yahoo 
servers, although shows non-Yahoo return addresses, and some portion 
have missing subject lines.

On further inspection, I find that the MISSING_SUBJECT, and in turn, I 
see by inspecting the message headers, that the subject header is 
rendered as "subject:" (with lower-case "s"), rather than "Subject:"

Is case-sensitivity causing that particular rule to miss, and is there a 
way to set a regexp to look specifically for that particular pattern?

I think I've found enough other rules hits to make for a distinct 
fingerprint, so that I can go after this stuff with a local rule, but to 
me, the case error in the header coming from a misconfigured robomailer 
seems to be a pretty reliable indicator.

Smith