This post mentions the basic steps you can take to help reduce the number of emails you send out that get blocked by spam filters -- hopefully resulting in a more rewarding marketing effort.
1. The right selection of words
Many spam filters work by analyzing the email based on its content and the words used. Many words -- such as free, sex, discount and so forth -- are very heavy spam trigger keywords. Your priority should be to avoid such words while keeping your newsletter as professional as possible.
2. Pay attention to your formatting
When formatting your email, keep it simple and professional. Excessive use of different colors, fonts, sizes, images and so forth will result in a higher spam filtering rate. Keep your email as clean as possible, and try to stick to a maximum of 2 or 3 different font types and sizes. Overly large sized fonts will surely add to an email being flagged as spam, as will too many images (or not enough text).
Try and use a short and simple stylesheet rather than using font tags excessively. Most spam filters don't appreciate a multitude of font tags and inline formatting, and the more primitive filters can't detect stylesheets so they will not penalize as easily.
3. Consistency is king
Use a template if you plan on sending newsletters consistently. This will make sure that all your newsletters look and feel the same. It will also add a touch of professionalism and branding to your newsletters.
Whilst not directly affecting spam filters, this will enable your readers to distinguish your newsletter instantly, thus not reporting it as spam accidentally. Some spam filters work by querying a spam server, whereas others report individual emails as spam. If your email gets reported as spam, then more than likely multiple spam filters will flag your email.
Being consistent with your timing of the newsletter also helps. For example, if you send a newsletter once per month (I personally don't recommend you send out any more than this, unless you've got something really interesting to say), then aim to send it out at the same time, on the same day each month.
Once again, your potential readers will learn to expect your email, adding professionalism and often improving open rates, also reducing accidental spam flagging as well.
4. Always use Double Opt-in
Always make your contact lists double opt-in. This means that when a user subscribes to your contact list, they will be sent an email with a link that they must click on to confirm their subscription.
This is very important because many people can accidentally enter an incorrect email address, or even the email address of someone else on purpose. When that person receives a newsletter they did not subscribe to, they will assume they have been spammed, and your newsletter (and possibly your web server) will be reported as spam.
5. Unsubscribe and Contact Information
Every newsletter you send out should contain a way for the reader to unsubscribe. Not doing so is illegal in some countries and is an instant sign of spamming. You should also display your contact information (Phone, Fax and Address) clearly, as this greatly increases confidence in your email and your company, as well as conforms to spam laws in the United States. Contact information also allows a potential customer to contact you if need be.
6. Test, Test, Test
The key to avoiding spam filters is testing. The first method of testing is to send the newsletter to multiple email accounts with existing spam filters. For example, I have a Gmail (http://www.gmail.com) account and a Hotmail (http://www.hotmail.com) account that I make sure I send my newsletter to. If the newsletter ends up in the junk folder, then I've got some work to do.
Conclusion
Avoiding spam filters when sending out legitimate newsletters can be a time consuming effort. However, as your contact list grows, it can also be a very beneficial exercise if you put more effort and time, considering the growth of contact list.