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Table Of Contents  CertiGuide to Security+
 9  Chapter 2:  Communication Security (Domain 2.0; 20%)
      9  2.3  The Web
           9  ActiveX

Previous Topic/Section  CGI
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2.4  Directory
Next Topic/Section  SMTP Relay
(Page 1 of 2)

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 recipient’s 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.

SMTP Relay

An SMTP relay is an SMTP server that accepts incoming mail from users in a different location, determines where it needs to be sent, then forwards that mail to the SMTP server at the recipient’s domain.

SMTP uses TCP port 25. (Why are we repeating these port numbers? Because you Need To Know them for exam success, and we want to ensure that we’ve given you every opportunity to see them .)

Why are SMTP Relays Needed?

Not everyone has an SMTP server running on his or her local network. If they don’t 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 employer’s 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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CertiGuide for Security+ ( on
Version 1.0 - Version Date: November 15, 2004

Adapted with permission from a work created by Tcat Houser et al. Version Copyright 2004 Charles M. Kozierok. All Rights Reserved.
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