We all know a few things about avoiding spam traps and filters.
Don’t use too much capitalization in your subject line. Don’t use words like Free, Offer, Money et cetera. Don’t use exclamation marks.

However, it isn’t enough simply to avoid the obvious words.

Your email will have to successfully pass through a number of filters at different points. All these filters together may use a combined list of literally hundreds of different words which they consider "spammy."

So before you send any email or newsletter, run it through a spam checker (run the words "free spam checker" through your favorite search engine to find one).

The spam checker will alert you to problems not only in your subject line but also within the body of the message.