One word comes to your mind: Spam. Professional copywriters for email marketing campaigns know that when the first thing to consider when sending emails to subscribers is to make sure the email gets delivered, then opened. Sometimes, you can't avoid using words like "free" or "opportunity".
Just FYI, these words usually go straight to the spam filter before it gets to your subscriber's inbox.
Don't waste precious words by writing them without giving a thought as to whether your subscribers will actually be able to receive them.
So how to get around this obstacle? Use words that are not likely to be considered by spam filters as spam. Maria Veloso in her book Web Copy that Sells suggests http://www.lyris.com/us-en/contentchecker. For starters, she also lists the most common words that are likely to be filtered by SpamAssassin and other anti-spam programs.
