Thunderbird does NOT Spell Check the Subject???

[luckysnafu]

I use Thunderbird for my email client and I just sent out an email to a client and cc'd my coworker. My coworker emails me back to tell me I spelled a word wrong in the subject, so I tested to see if the spell check would have caught it and it doesn't even check the subject. What's up with that? Does outlook check the subject?