Subject lines, titles and slogans rule our lives. First impression. That’s what really counts in online and offline marketing as well. Your articles won’t be read, your emails won’t be opened, you won’t get noticed without a killer subject line, title or slogan. But will FREE help you to stand out? 
Is FREE a magic word?
Maybe it’s the favorite word of many, but do you honestly believe that you will get anything for free? I don’t, but still “free” is an often debated term in email marketing. Should you use it in your subject line? According to Mailchimp, you should avoid FREE in subject lines, since it will help you end up in spam. Many others say that (Smartinsights, Returnpath, Hubspot, Litmus) FREE is no longer a bad word and it won’t make a difference in the deliverability of your email at least. There may be some situations when FREE will work for you, but it will highly depend on your audience and email marketing history.
Build trust. Be honest. Don’t fool people with FREE!
If you were always sending useful stuff to your subscribers, and you never fooled them, a relevant FREE”in the subject line is likely boost your open rates. We all love FREE. We just hardly believe it, since it usually turns out that we have to pay for FREE things, one way or another. Can you remember the last time when you got something for absolutely FREE? I bet you do, since there are only a few occasions when it happens.
Advice: A/B test your subject line
Only testing will tell you if it helps or not in your case. If possible please take into account not only open rates, but click-through and conversion rates as well. Open rates may fool you badly, since it is usually not the metric you should concentrate on!
