There is a class of Email Spam coming with empty subject (sometime no To: fields) and generally, those are not flagged by spamfilters as such (bummer!) polluting your INBOX. They could be handled in your mailer with the following trick however:

In Thunderbird 3.x or above, add a filter on "Subject" "doesn't contain" and select (add one line for each) a, e, i, o, u, y, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and ":", "&", ".". Select "Match all of the following" option. You may want to add a few more letters such as "t", "n", "s", "h", "r", "d", "l", "c" and "m" representing the top most frequent consonances in English.

Notes:

• I suggest you move the Emails to low priority folder rather than deleting them since there are always careless people sending you empty subject Emails (personally, I think their communication is spam but that is my own viewpoint) ...
• Verify this filter really works for you - Filter is to be avoided if you receive Emails in foreign languages not following Latin encoding  ...
• Note that I wrote Thunderbird 3.x or above (if you see this filter in 2.x, strange matching results will appear / you have been informed)

This is what appears in my msgFilterRules.dat for Thunderbird 3.1.2 and moving to a folder named ETrash.

name="Empty subject"
enabled="yes"
type="17"
action="Move to folder"
actionValue="imap://username@mymailbox.bnl.gov/ETrash"
condition="AND (subject,doesn't contain,a) AND (subject,doesn't contain,e) AND (subject,doesn't contain,e) AND (subject,doesn't contain,o) AND (subject,doesn't contain,u) AND (subject,doesn't contain,y) AND (subject,doesn't contain,0) AND (subject,doesn't contain,1) AND (subject,doesn't contain,2) AND (subject,doesn't contain,3) AND (subject,doesn't contain,4) AND (subject,doesn't contain,5) AND (subject,doesn't contain,6) AND (subject,doesn't contain,7) AND (subject,doesn't contain,8) AND (subject,doesn't contain,9) AND (subject,doesn't contain,:) AND (subject,doesn't contain,&) AND (subject,doesn't contain,.) AND (subject,doesn't contain,t) AND (subject,doesn't contain,n) AND (subject,doesn't contain,s) AND (subject,doesn't contain,h) AND (subject,doesn't contain,r) AND (subject,doesn't contain,d) AND (subject,doesn't contain,l) AND (subject,doesn't contain,c) AND (subject,doesn't contain,m)"