1.  The email subject line must get the reader's attention and inspire them to want to view the contents of the email.  If the subject doesn't speak to the reader, the likelihood they will delete the message rather than viewing increases.  Further, people are just as likely to report emails as spam if they perceive the topic to be something that isn't currently relevant to them, even if they previously requested more information on the topic, so being relevant/timely and communicating that in the email subject is critical to having emails viewed consistently.

2.  The email body (content) must present a brief yet compelling case that inspires the recipient to take the desired action (=buy, sign up for newsletter, etc.).  The body of the email needs to educate, convince and inspire to act in a relatively short time.  People are more likely to click through links in a marketing email if they can see enough to know they want to view all of the information by what their email client preview reveals to them, and if they can learn what they need to know to decide whether or not to act without having to scroll down within the email.  That really doesn't leave much room to make the case, so brevity and simplicity are key.

3.  The email characteristics (subject line, header, image-to-text ratio, reply email, etc.) must pass spam filtering tests used by email hosts.  If the email doesn't pass muster to get to the recipient's Inbox, then sending it is a waste of time.  To make sure emails meet accepted standards requires more sophistication then it used to, so small businesses are well-advised to use online email list management tools that make it easy to incorporate these requirements and test emails for compliance before sending.

4.  The graphics must be attractive and used as an enhancement for the email message, but the message itself must be viewable even if the graphics are not seen (i.e., if the recipient uses Outlook, which blocks automatic image downloading for privacy purposes).  If the alt text isn't set up to deliver the message so those who don't download the images can receive it, chances are the message will be deleted.

5.  As part of the ongoing shift toward giving consumers more control over the communications they receive, the ability to unsubscribe or modify one's subscription preferences remains essential to maintaining your mailing list.  When customers are not given the option to opt out, or if their efforts to opt out don't work, they will report emails as spam in a heartbeat.