Manage Your Subject Line

Subject line will be a huge determinant to your email's ability to get into the inbox. You should not write an email subject line that is significantly longer than your header because the longer your email subject line is, the more likely it would be flagged as spam.

Remember three primary rules:

•     Shorter is better! (But not less than five or six words)
  
•     Don't rely on words that are included in the "spam words" list in the next section of this book.
  
•     Avoid using all caps or special characters in your email. They may end up resulting in having you flagged as spam.
  

You can get away with using "spam words" in your subject line if you have a great sender reputation and use them with moderation. However, if your test email goes to the spam or junk folder and you have a spam word in your subject line; your first test fix should be to remove it.

Write Content that Isn't Full of Spam Words

This rule, as well, may seem fairly obvious. However, you'd be surprised how many emails are sent daily and then end up in the spam or junk folder because they were loaded down with words like "free" or "credit." Writing content for email marketing can be a very specific process that's not different from writing for a Search Engine Optimization page in the fact that you need to write to a certain algorithm. There are many free online tools that will scan your email content and give you a score or a color-coded rating regarding the likelihood that you will be flagged as spam. While these tools are imperfect, they can give you a red flag if you are clearly in the danger zone.

The basic rule of thumb is to understand the words and types of language that most email service provider spam filters will consider likely junk mail. Then simply write in a manner that doesn't incorporate those words or style. It may mean that you need to think a little harder while writing your copy, but that effort will be worth it when you hit the inbox. After all, it doesn't matter how persuasive the word "free" would be with your subscribers if they never actually see the email.

Avoid Emails that Are One Large Image: Minimize Images Overall

Emails that are one large image have a high chance of being flagged as spam if you don't already have a good sender reputation. Using a large image to encompass the entire email is a frequent trick of email spammers. If the entire content of your email is in an image file, then the email spam filters have nothing to spider in terms of content and can't figure out if your message is junk or not. We've all seen emails get delivered to our inbox that are one large image. However, your deliverability chances decrease if your email is one big image. As we noted earlier, even if you hit the inbox, your conversion percentage will be decreased as well.

While images are an important part of any email template, the more images you use the more you may experience spam filter issues. If you are having a difficult time making it into the inbox, then one of the first steps you'll want to explore is to reduce the number of images in your email template.
Minimize the Use of Red Fonts and Huge Headline Size Fonts

Red fonts and huge headline size fonts have also been shown to cause spam filter issues, though not as frequently as many of the other issues noted here. As a general rule, it's just a better idea to avoid using red fonts (pick an off-red color), huge headline fonts of more than sixteen pixels or a combination of both. There are plenty of design options that still give you great flexibility without using those font sizes and colors.