Using Urchin Tracking Module (UTM), we can easily segment the Mailer campaign traffic by medium, source and the visitor mailer content engagement. By customizing Mailer "Source" through UTM, Google Analytics unable to capture the email client details (like gmail, yahoo, hotmail, outlook). To solve this issue, it is better to pass Email client details in UTM Source itself.
For example, if you are running promotional offer campaign, pass mailer campaign & with Email client detials in UTM_Source [UTM_Source=ProMailCamp-Gmail]
Advantage – By doing so, we can track the Email Client Engagement behavior.Google+