We have a video of this process available here.
Use the "email" Retargeting Audience method to create an image pixel that can be added to an HTML e-mail template to cookie people when they open the e-mail. This method requires the user to view the e-mail in HTML format in a supported web browser with images enabled. The user will also need to visit a webpage with our tracking tag installed before they will be added to the Audience. For example:
Scenario: You use MailChimp to send a weekly newsletter to 5,000 people and want to retarget people who open the newsletter with ads.
Solution: Create an "email" Audience named after your newsletter. You will be taken to the "detail" page for the Audience where you will get the pixel code to add to your e-mail list template in Mailchimp. The next time you send an e-mail, anyone who visits or has visited your website opens the email in HTML format will be added to the Retargeting Audience.
Note: Due to how Gmail handles cached images, users who open emails with our pixel in Gmail may not be cookied.