For email campaigns your newsletter tool needs to support A/B tests (assigning your recipients into different groups). You can then send different newsletters to your visitors and see how the different newsletters affect the browsing behaviour on your website. This is only recommended when you have many users that click on your newsletter.
To track how different ad or email campaigns affect the browsing behaviour on your website, you only need to add two URL parameters to your campaign URLs. Read more about this in the guide for running experiments in a campaign.