Question:

Why do Email Clients render the CRM Approved Email differently?

Answer:

Email clients and apps can behave differently when displaying HTML and Styling.
For troubleshooting, please consider using the following resources.

• Litmus (https://litmus.com/) allows users to test email designs on many email clients, and to do spam filter testing.
• Campaign Monitor (https://www.campaignmonitor.com/css/) provides a guide on CSS, including information on CSS support by major email clients. 
• Campaign Monitor also provides guidelines for HTML Formatting: https://www.campaignmonitor.com/dev-resources/guides/coding-html-emails/
• Campaign Monitor also has a guide for mobile optimization of responsive email: https://www.campaignmonitor.com/dev-resources/guides/mobile/
• Zurb has a guide to responsive design, with examples: 
  http://zurb.com/playground/responsive-email-templates
• You can also do testing locally on versions of email clients or apps.

Related Documentation:

CRM Help Documentation: Resources for Creating Approved Email Content