18.104.22.168 SMTP Relay
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An SMTP relay is an SMTP email server that accepts connections from users, usually in a different location from the server, wishing to send email. It receives the message they wish to send, and then relays (sends) that message on to the SMTP server that delivers mail for the recipients domain. SMTP uses TCP port 25. Sites wishing to make connections to other SMTP servers (« send mail ») must have outbound access to the SMTP port, and those wishing to receive connections from other SMTP servers (« receive mail ») must have inbound access to their SMTP server enabled. Practically speaking, this means that access to port 25 on your mail server should be provided for both inbound and outbound connections.
Not everyone has an SMTP server running on his or her local network. If they dont have a local SMTP server, they would use an SMTP relay to get their outbound mail onto the Internet. Many dial-up Internet users simply use the SMTP relay provided by their ISP. Telecommuting users with dial-up Internet connections may wish to send email via their employers SMTP server, so that the email headers show it originated from inside the company.
SMTP relays are a fine thing. Without them, Internet users around the world who depend on clients such as Eudora and Outlook express to send mail would never be able to get a message out to the Internet.
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