The truth is we are naturally wired to be a bit skeptical. While that free offer sounds good, your first inclination would be that it's too good to be true. And who could blame you, most of the time we go through life believing that nothing in life is free. Therefore, why would some random company email me an offer with the subject line "Get a FREE T-shirt - Details Inside!" You'll be convinced there is some catch requiring a $50 purchase, or attending at least five of their informational sessions before you even click on it. Then one of three things happens.
A. You'll either delete it and never give it a second thought
B. You'll see it in your spam folder and ignore it because you figured it was in there for a good reason.
C. Open the email only to unsubscribe from the list.
If one of the three happens, you're losing potential costumers and all that time you spent carefully constructing the perfect email would be wasted. Instead of using the word "free", reference an offer that can't be passed up or make mention of the easy way to "earn" discounted swag. You might find you'll get a lot more clicks, opens and maybe even sales.