You can insert a link into your campaign that when clicked upon will start an email from the contact to a specified email address with a specified subject line.
The option to add links (hyperlinks) appears on the ribbon whenever you are working on text or images. When adding links to text, first type the text into your email, then select that text before clicking the Link button and choosing Email from the menu of the window that pops up.

A link to an email address can have the following aspects set in the Add a link screen:
The text preview shows how this link will appear in the campaign.