Many spam filters work by analyzing the email based on its content and the words used. Many words — such as free, sex 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.
Later in this article I will show you a technique that I use to help me detect words that could trigger spam filters that I may have missed.
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.
Consistency is key
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.
Always make your mailing lists double opt-in. This means that when a user subscribes to your mailing list, they will be sent an email with a link that they must click on to confirm their subscription.
The key to avoiding spam filters is testing. The first method of testing I use 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.