A system for transmitting electronic content over a computer network is provided. In one technique, client devices that visit a website send, to a content delivery service, interaction data that indicates that the client devices interacted with the website. Later, a content provider of the website creates a content delivery campaign with the service or establishes (or renews) a retargeting segment for an existing campaign. The content delivery campaign indicates that certain content is to be delivered to entities who have interacted with the content provider, such as by visiting the website. The content delivery service uses the interaction data to identify candidate entities who satisfy targeting criteria of the retargeting segment. If a candidate entity does satisfy the targeting criteria, then the candidate entity is added to the retargeting segment, even though the candidate entity may not have interacted with the website after the retargeting segment was created or renewed.