By default, “Email” is not considered as a “source” in Google Analytics.
The correct answer is: Email
Explanation: “Email” is not considered as a source in Google Analytics. “Email” comes under the “Medium”.
The source can be the name of the referring site, name of your marketing campaign, or direct (users that typed your URL directly into their browser, or who had bookmarked your site).
Possible medium include: “organic” (unpaid search), “CPC” (cost per click, i.e. paid search), “referral” (referral), “email” (the name of a custom medium you have created), “none” (direct traffic has a medium of “none”).