Pixel Size: HTML emails should fall into the range of 500-650 pixels wide as this is the size of the message window for popular web-based email services like Google and Yahoo! mail-- varying on the users' screen resolution settings. Emails that are wider than that require contacts to scroll horizontally to view all of the information. 

Hierarchy: Important information and links should be placed towards the top of the message, helping the reader focus on the main message without much scrolling. Avoid taking up valuable real estate at the top of the message with unnecessary links or images.

Tables: Some inboxes do not support modern web tags such as <div> and <p>. Use tables in the HTML code to control placement of text and images.

Image Size: Images should be a web safe format, JPEG, GIF, and PNG. Keep images under 40 KB and the entire message under 100 KB for optimal load times.

Embedding Images: One of the most common mistakes involves improperly embedding images into messages. Images cannot simply be copied and pasted into the body of an email message. The images must reside online and the URL must be embedded in the message in order for the image to render correctly. Listrak includes a media library for you to easily upload, store, and reuse images as needed.

CSS: CSS should be placed inline on any HTML elements and table structure in the message. Although cascading style sheets are commonly used in web design, avoid using external or internal CSS in the header, as it can render incorrectly in many email clients, or simply get stripped or overwritten.

Testing: Test your emails in different email clients prior to sending the message. Determine if there are any rendering issues that need to be fixed prior to deployment.

Avoid using image maps.

Avoid using Java, JavaScript, frames, ActiveX, cache busters, or dynamic HTML.