An SMTP relay is a computer that routes incoming and outgoing emails to a specific destination. There are two types: open relay and authenticated relay. SMTP relays often use authentication to prevent harmful emails, and can be hosted on a dedicated machine for added security.
A simple mail transfer protocol (SMTP) relay is a computer connected to the Internet and running software that allows it to act as an SMTP server with the intention of routing incoming and outgoing electronic mail (email). exit to a specific destination online. An SMTP relay server is capable of accepting emails from a remote source and delivering the emails internally, if the destination is the server itself, or forwarding those emails to other SMTP servers if needed. In addition, this forwarding can be configured in several ways to prevent malicious email or junk mail from being processed. There are two main types of SMTP relay servers, the first is an open relay by which any computer anywhere in the world can connect and send mail through the server. The second type is an authenticated relay, through which any system connecting to the SMTP relay must supply at least one valid username and password combination, although multiple usernames and passwords may be required for each operation.
When an e-mail message is composed and sent by a user, the message is usually passed through an SMTP server to reach the recipient. If one SMTP server contacts another SMTP server and transfers an email to it, the SMTP server that receives the email is known as a relay SMTP, because it is relaying the email. The forwarder does not have to be the destination server and can act as a gateway or intermediate step between the source server and the destination server. If the relay is the destination server, received e-mail messages can be passed to local software so that they can be retrieved by client programs as needed.
Many SMTP relay servers use authentication to determine which incoming mail traffic should be processed and which messages should be rejected and left for other servers to process. Often, an SMTP relay will only serve incoming or outgoing messages that have some relation to the Internet domain it is connected to. This prevents anonymous or random users from using the server to send emails that may be harmful or unwelcome. If no authentication is required, SMTP relay will simply process all incoming messages as best it can.
In some situations, an SMTP relay server can be hosted on a dedicated machine. This has the advantage of isolating the server from the rest of a larger network. It also allows a relay to act as a front-end filter, stripping malicious emails or stripping messages of their attachments before passing them to other SMTP relays for actual processing.
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