When a hotlink is included in an email, a clickthrough occurs when a recipient clicks on the link. clickthrough tracking refers to the data collected about each clickthrough link, such as how many people clicked it, how many clicks resulted in desired actions such as sales, forwards or subscriptions.

Combining Email Marketing with Other Types of Marketing
Whether you choose to do your marketing online or offline, one thing does not change. This is the concept that no single marketing strategy will be as effective as a combination of two or more marketing strategies.