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Simple Mail Transfer Protocol (SMTP)

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP) controls the way e-mail is transported and delivered across the Internet to the destination server. SMTP receives and sends e-mail between servers. The SMTP service is installed by default with the POP3 service to provide complete e-mail services.

The SMTP service is automatically installed on the computer where the POP3 service is installed to allow users to send outgoing e-mail. When you create a domain using the POP3 service, the domain is also added to the SMTP service to allow mailboxes in that domain to send outgoing e-mail. The SMTP service on the mail server receives incoming mail and transfers the e-mail to the mail store.

E-mail relay

E-mail relay occurs when users who are not members of the e-mail domain use a mail server with SMTP to send e-mail. SMTP mail servers that are not configured to prevent open relay are often abused by those who want to send large volumes of unsolicited commercial e-mail. The Microsoft SMTP service is configured by default to prevent e-mail relaying.

If you want to enable e-mail relay, however, you have the following two options, depending on which authentication method you are using:

  • If you are using Active Directory integrated authentication or local Windows accounts authentication, you can configure the mail server to require authentication before accepting outgoing e-mail.

    For more information on configuring the mail server, see Configure the Simple Mail Transfer Protocol service for e-mail relaying. If you configure the SMTP service to authenticate users trying to send outgoing mail, you must also configure the users' e-mail clients. For more information about configuring e-mail clients to allow relaying, see Configure e-mail clients for e-mail relaying.

  • If you are using encrypted password file and want to configure e-mail relaying, you must configure the mail server to allow relay based on either Internet Protocol (IP) address or e-mail domain name.

Note

  • When an e-mail is undeliverable, the Simple Mail Transfer Protocol (SMTP) service returns it to the sender with a non-delivery report (NDR). If the NDR cannot be delivered to the sender, a copy of the message is put in the Badmail directory. To prevent the possibility of the operating system running out of disk space (if the Badmail directory becomes too large), you must move the SMTP Badmail directory to a volume other than the one on which the operating system is installed.