Here's a great free resource: www.ezinecheck.com. Paste the text of your email messages into the online form and the application offers up what to fix.

The E-zine Checker helps your e-zine avoid the spam traps by checking for commonly used spam words, words in all capital letters and excessive use of words with symbols in them. Every time an email has a word or phrase that triggers a spam-filtering flag, it adds a penalty point. If your score ranks high, your e-zine is likely to get caught in the spam filters. If your score is low or zero, your e-zine is likely to get delivered into your subscribers' inboxes. The goal is to get a spam score of one or zero.

Source: MarketingProfs - Case Study: How to Beat the Spam Filters