Format your emails with Cascading Style Sheets.

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Cascading Style Sheets (CSS) can be used to style your emails; create professional branding, specific marketing and event styling

You can add a CSS to every email / for an individual user email account / email template / or specific email.

1. System wide email CSS - Once you add a system wide email CSS all new emails will use this style.

Go to : Admin > Setting > select tab named 'Email' and set your CSS

 System email CSS

With this example the email editor for all the users would look like the below.

Email with system CSS 

2. User account