First of all appreciate your share, like your blog, the hope can share with you more wonderful blog,welcome to Canada GooseParka, Canada Goose Coats and Canada Goose UK.

"Free" is not a bad word, but it is important to build trust with your mailing list so that "free" gives you strong CTRs. No cache, and people will delete your message even if it gets to you.  
 
Side note, but some filtering is smart enough to realize that "free" isn't necessarily bad. For example, in GMail, if I always open Godin's emails, and I have "free" set as a word to delete, his email with "free" in the title will still get through because I've shown GMail that he's a trusted sender to me. (Of course I can change that by marking something as SPAM or by lowering priority in Priority Inbx, but you get the idea)

Great discussion everyone! I agree that you should not use "free" in any of your subject lines. I've experimented using "free" in the subject line and my click-throughs were always lower, presumably because of firewalls and spam filters blocking the message.

Seth's point may have been accurate, but it was very naive. The wording affects spam filters so many of his messages that went out may have been blocked. It is really as simple as that.

I personally wouldn't do it. I think it degrades your professionalism.

I'm a subscriber of Seth's and remember receiving this email, but don't recall whether it went straight to junk. I agree, I personally would not use the word "free" in a subject line. Too risky.

I agree with the other comments. While Seth's reply makes sense, it still doesn't answer the question of whether "free" in the subject line is an acceptable email practice from a SPAM perspective. 
 
Seth Godin has enough name recognition and is on plenty of "Safe Lists" that he can take a chance and maybe lose a thousand or so of his millions of subscribers. But for a small businesses, it still doesn't seem like a good idea to use "free" in your subject line and risk being Black Listed.

Question: what if the subject line does not have "free" but the email itself does. Does this affect spam filtering also?

But how did it even get through the spam filters I wonder.

I would say that it wouldn't be worth it to include "free" in the subject line as the email with this article went straight to my spam folder which usually never happens. Why take time to compile and send out an email that most likely won't even reach your target audience. I'd rather try to come up with a more compelling subject line that will reach everyone on my contact list. I wouldn't count on anyone configuring their email settings to allow their inbox to accept emails from a certain address just so I can put spam words in an email subject line.

How did the spam filters know Seth was really offering something for free?