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 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:
