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A simple mail transfer protocol relay (SMTP relay) is a service that is used as a means to transport email messages in between different email hosting services, servers and/or domains.
An SMTP relay is primarily used when an email needs to be delivered to a domain different from the user’s domain. For example, if an email is being sent from email@example.com to firstname.lastname@example.org, SMTP relay is required to transmit messages between each domain. SMTP relay is generally a third-party application or service that mediates the process of sending, delivering and fetching email messages between different email hosts.
SMTP relays are also used in email composing applications such as Outlook. Users can configure their email address on the software and are able to send emails from multiple email addresses (of the same or a different domain) without the need to consider domain, ISP or organization.