This is a complex question with a number of potential answers. There are many various factors that determine if any given email is junk or not. 

While we don’t really think about it, there’s a whole world of journeying in store for an email after we hit click the send button. It’s easy to lose sight of that because we don’t have an active role in anything that happens to the email after we click send.

Different state laws govern spam and its legal definition in the United States, and there is also the federal CAN-SPAM Act of 2003.^[1] To combat spam, many online entities use spam filters that target and remove emails that, at the very least, meet those legal criteria.

A spam filter is a software program that scans emails as they flow by. It’s programmed with a specific set of criteria for what spam looks like, and pulls email that meet enough of those criteria out of the flow. All quality internet mail servers are equipped with a spam filter, plus they are often installed on network servers and on individual PCs.

Every time one of your emails is marked as spam by a recipient, more scrutiny is placed on your future emails by the inherent filtering processes. 

Use the following suggestions:

1. DON’T SHOUT IN ALL CAPS IN YOUR SUBJECT LINE!

2. D o n t   p u t  g a p s  b e t w e e n  l e t t e r s  a n y w h e r e !

3. One exclamation marks works fine, don’t go crazy!!!!!!!!!!!!!!!

4. Stay away from words and phrases that sound gimmicky: Buy Now, Click Here, Free, Free Print, Lowest Prices of the Year, Money Making, Get Paid, Satisfaction Guaranteed, Low Price, Huge Studio Sale, etc.

5. Encourage subscribers to whitelist/add you as a contact.

6. Personalize your 'To' field with the recipient’s name.
   See this FAQ: Add a Personal Salutation

7. Be sure your 'From' name is relevant to your art website to prevent confusion.

8. Aim for a high text to image area ratio.
   Having more space in the body of an email taken up by images than by text raises lots of red flags. We recommend a maximum of 40% image coverage. Have several, smaller images with clear, descriptive alternate texts.

9. Be very careful about the quality and quantity of links in your content.
   Keep the number of links minimized and don’t use url shorteners.

10. Avoid putting the full URL to outside domains in your newsletter. Not only does it look horrendous but if any of the domains get blacklisted (which is completely out of your control) your newsletters will start going to junk. Be sure to use a hyperlink in a word instead. See this FAQ: Hyperlink in Newsletter ~ Link Color

11. Give a clear and obvious opt-out feature from future emails.
    (Included in FASO Artful Mail)

12. Reach out to inactive subscribers and keep your contact lists whittled down to those who want to hear from you. This can reduce the likelihood that folks that have lost interest will just mark you as spam to stop the emails because it’s faster than unsubscribing. A smaller, more engaged email list always trumps a large list full of unengaged users.

13. Stay In Touch!

    Sending emails once every two or three months may be more detrimental than sending multiple emails weekly. Why? Your Customers might forget all about you.
    If subscribers fail to recognize the From field, they may delete your message, hurting your “open” ratio.
    They may forget that they subscribed and generate negative feedback by marking messages as spam.
    There are lots of good reasons for maintaining a steady flow of communication, rather than relying on infrequent, massive “blasts.”

14. Make sure you are recognized as a legitimate sender. 

    Follow best practices and use a double opt-in process.
    See this FAQ: Use Double Opt-In for Newsletter Subscribers
    NOTE: Double Opt-in is automatic for all artists that signed up for a FASO membership plan after November 6, 2013.

    Make sure your recipients click on "Always display images from this sender" which will let the email service know that you are a trusted sender.

15. Make sure  you are using a branded email address for newsletter sending.
    See this FAQ: Use Domain Email for Newsletter Sending

Some of the above content is from this Ninja Form article:
 Why Your Email Gets Sent to Spam & Quick Tips to Keep Out of the Can!

Also see
11 Reasons Why Your Emails Go in the Spam Box (and How to Make Sure They Don’t)

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