Following are the rules that must be followed to avoid being marked as SPAM −
Using phrases like “Click here!” or “Once in a lifetime opportunity!”
Excessive use of exclamation points!!!!!!!!!
USING ALL CAPS, WHICH IS LIKE SCREAMING AT THE TOP OF YOUR LUNGS VIA EMAIL (especially in the subject line).
Using bright red or green colored fonts.
Using bad content. This one’s broad, but important.
Coding sloppy HTML, usually from converting a Microsoft Word file to HTML.
Creating an HTML email that’s nothing but one big image, with little or no text. Spam filters can’t read images, so they assume you’re a spammer trying to trick them.
Using the word “test” in the subject line. Agencies can run into this issue when sending drafts to clients for approval.
Sending a test to multiple recipients within the same company. That company’s email firewall often assumes it’s a spam attack.
Sending to inactive lists. These are lists which have not engaged in the campaigns through opens and clicks. Because subscriber engagement is a huge part of getting emails into the inbox, when an ISP sees low engagement rates they will often begin to bulk the campaigns to the spam folder. Then they will block the domain and IP addresses used to deliver those campaigns.
Sending to stale lists. Permission generally goes stale within about 6 months, so if your subscribers haven’t heard from you within that timeframe, you’ll need to reconfirm.
Include a Text version of your email if you are sending html emails.
Use Spam checkers before sending your emails.
Maintain a Good Text to Image Ratio.
Make Sure Your DKIM, SPF, Sender-ID, and Domain Keys are set up properly.
Avoid large attachments and certain attachment types.
Before sending emails out to your entire list, it’s worth the time to utilize a spam checking service. Websites like MailingCheck.com offers a free downloadable tool for Windows that uses a Spam Assassin to check the email.
If you prefer to avoid downloading any software, you can send the email to the IsNotSpam.com service and they will also check a few other items important regarding the email deliverability. Alternatively, Mail-tester.com uses a form-based solution to test your emails.