SPAM filters have become increasingly complex in order to identify which emails are from legitimate senders which are not.  Spam filters look at a long list of criteria to decide whether your email is spam.  Also, spam filters are very private about why your email passed or failed to try to prevent spammers from figuring out their techniques.

Typically, there is not one single change that can be made in order to let your email pass a spam content filter.  Instead, most filters use a scoring system, if you score high enough, they block your email.  Having !!! in the subject may be 10 points, and having a high image to text ratio could be 24 points!  Once you are over a certain score, your email is blocked.  There are 100's of spam filters, each with their own unique settings. 

Reasons a message could land in the SPAM or junk folder

Subscriber Engagement

• AOL, Gmail, Hotmail, Yahoo!, and other major email clients use subscriber engagement to filter spam
  1. The more your recipients open, click, and read your messages, the more highly engaged your audience
  2. The more engaged your audience, the more likely your messages are to land in the inbox
  3. If your subscribers are not engaged, the message could be filtered as spam
• Email providers don't share exact filtering data with senders. But by tracking your campaigns' opens, clicks, and unsubscribes, you'll be able to gauge how your subscribers are engaging with your messages

Subscribers click "report spam" button

• One of the greatest marks against you if you're having trouble getting into the inbox will be if your email content has been previously flagged as spam by your subscribers
• If a lot of your recipients click the "report spam" button against your messages, their email client can start to block your campaigns and deliver other messages straight to the spam folder
• If subscribers move emails out of the spam folder, that's a positive indicator and can help ensure future emails that are similar are delivered to the inbox instead.

Subscribers No Longer Wish to Receive Messages

• A frequent sending pattern might be tolerated initially, but eventually subscribers may tire of the frequency and start marking your mail as spam
• To prevent this, you may want to adopt engagement based sending

List Hygiene

SPAM Traps

• Spam Traps are email addresses that are used by a blacklist to test if the email address was obtained by a bot crawling for email addresses.  If you send to a SPAM trap email address, the SPAM filters will know that you did not receive this address through a valid opt-in
• Purchased lists can contain many dead emails and spam traps that quickly inform mailbox providers that you are sending unsolicited emails
• To avoid including a SPAM trap email in your mailing list, use an opt-in process to obtain your subscribers and do not buy lists

Unsubscribes

• If the unsubscribe link is not prominently displayed in the message or the message footer, some subscribers choose to mark as SPAM instead of unsubscribing

Subject Line

• Misleading claims could make the message look suspicious: the subject line needs to correlate with the content of the message.
  Example: the subject line states that the recipient has won a prize, while the copy lists conditions that have to be met in order to claim it.
• Your subject contains trigger words or excessive punctuation (FREE!!!!)
• Exclamation marks are especially risky in email subject lines

From Address

• A free email address as your from address can trigger SPAM filters
  
  1. Avoid using a free/personal email address such as @hotmail.com or @gmail.com
  2. Instead, use an email domain for the company or organization for which you are sending email
  3. Make sure to use a working email address that can receive messages
• Your from address needs to be recognizable and trustworthy to external audiences: if subscribers do not recognize the address, they may mark the message as SPAM before opening it
• No from address or noreply@ since this looks like spam and will increase the likelihood of your email being filtered.
• Frequent changes of from address names
• Obscure from address field names or from addresses that do not exist

Message Content

Word Choice

• There is no complete list of spam trigger words, but you can try testing various text and avoid the risky words listed below.
• Be careful with words associated with the language of sales. If overused, they may trigger spam filters and route your emails to junk folders. Risky words include: prize, free, bonus, buy, purchase, order, guarantee, credit card, sex
• Anything about getting money, paying less money, or money-back guarantees may be marked as SPAM
• Excessive use of "click here."  Make your call-to-action links more specific to avoid filters.
• Don't forget to use spell check; misspellings are yet another spam indicator in your email content

Font Styles

1. Font Colors

   • Do not use too many different font colors (especially red or green)
   • Since red is a loud color and can be hard to read, it may cause your message to be caught in an email filter

2. All Capital Letters

   • Using all capital letters MAKES IT LOOK LIKE YOU'RE SHOUTING, makes your email look like spam, and will dramatically increase the likelihood of your email being filtered
   • When emphasis is needed, use a maximum of one word per sentence in all capitals, never a whole sentence

3. Extraneous Styles

   • Make sure your content does not include styling from Microsoft Word. If you are pasting in content from Word, you will want to use the “Paste from Word” button in the message editor to avoid having extra styling tags pasted along with the text

4. Font Size

   • Using extra large font size could trigger a filter.  Ordinary font size is considered 10-14px

Excessive Punctuation Marks or Symbols 

• !!! or ??? or $$$ are likely to trip email filters especially when used in conjunction with spam-like words and capital letters.
• One exclamation mark per sentence is enough

Links

• Spam filters check the links in your message. If you link to a domain that has a poor reputation you will be penalized.
• Check that the links in your message are valid.
• As a best practice, please avoid shortened URLs in emails because spammers frequently abuse them. Some shortened domains have been placed on some of the most widely used block lists, which means that emails containing these links can be blocked by a spam filter
• Linking to too many different domains in an email could make the message look suspicious
• Additionally you should avoid linking to URL’s that contain folders with 1-2 characters, because some filters will see that as a bad thing

Unsupported Content

• Javascript, form code, or embedded video within your email could flag the message as SPAM
• Sending attachments with the message

Images

• Good text to image ratio: it's suggested that for every graphic, include at least two lines of text
• Do not send any image-only emails
• Optimize images: this will shrink the overall size of the message

Message Length

• An email with no content in the subject and body of the message might be classified as spam.
• Keep emails short because too much copy is another red flag for spam filters.

Email Versions

• Include both a HTML and text version of your email.  Not having a text only version of the message could trigger a SPAM filter
• Note: our system automatically generates a text version from the text inserted into the message tab of the message editor.  Just make sure that if your message contains only images, you add some text to the text only tab.

Incomplete Information

• Your physical address is required by law. Always include your reply email address and your Web site URL, if you have one. Depending on your business, you may decide to include your phone number as well.
• Your physical address is added by default to the message footer.  If you customize the footer, please make sure to add the physical mailing address to this footer of elsewhere in the message content