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email client

n. A computer program used to manage email.

Wikipedia
Email client

In Internet, an email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email.

Client is meant to be a role. For example, a web application which provides message management, composition, and reception functions may internally act as an email client; as a whole, it is commonly referred to as webmail. Likewise, email client may be referred to a piece of computer hardware or software whose primary or most visible role is to work as an email client.