Add email link to custom or letterhead HTML templates

To find the email template you want to edit, go to:

Lightning Experience: App launcher | enter  Email Templates 
Salesforce Classic: Setup | enter Classic Email Templates in the Quick Find box.

1. Select an email template.
2. Click Edit. 
4. Insert the email address you want to hyperlink.
5. Highlight the email address, then click on the link button in the toolbar at the top of the template (this symbol is used to insert hyperlinks)
6. Then finally, click Save.

For HTML with letterhead email templates:

1. Edit the HTML version of the email  template.
2. Select the email address you need to change to an email link.
3. Click on the link button in the toolbar at the top of the template (this symbol is used to insert hyperlinks).
4. Enter "mailto:john.smith@example.com" in the pop-up window.
5. Click "Save".

You can also select "John Smith" instead of the email address, and follow the same steps. This way your name can also appear as an email link.

For custom HTML email templates:

1. Edit the HTML version of the email template.
2. Instead of "john.smith@example.com", enter this :
<a href="mailto:john.smith@example.com">john.smith@example.com</a> 
3. Click "Save".

And if you want your name to appear as an email link instead of the email address itself, you can replace this with :
<a href="mailto:john.smith@example.com">John Smith</a>