This article was previously published under Q230235
This article briefly explains how to set up and configure the Microsoft Simple Mail Transfer Protocol (SMTP) service to act as an SMTP relay server after installation. The SMTP service comes in Internet Information Server (IIS) 4.0 and Microsoft Commercial Internet System (MCIS) 2.0.
If Microsoft SMTP service is not configured as an SMTP relay server, then e-mail clients using it may receive the following error message when sending out mail:
550 Unable to relay for email@example.com.
To configure the SMTP service using Microsoft Management Console (MMC):
Go to the SMTP Site Property page and select the Delivery Property page.
If you are using a Smart host to handle your Domain Name System (DNS) resolution, type the Smart host's IP address in the Smart Host field.
Next, if you are attempting to do your own DNS, click to select the Attempt direct delivery before sending to smart host check box.
NOTE: DNS must be set up properly in the TCP/IP Protocol Property pages. To relay messages to the Internet, be sure that your ISP's DNS server is located on the DNS tab or that you are using a DNS server that has access to Internet Mail Exchanger (MX) records.
If this is a new installation, go to the SMTP Site Property page. Verify that the Fully Qualified Domain Name (FQDN) is the SMTP domain name you want to use for your default site. This will appear in your SMTP conversation "helo" banner.
Select the Domains object. For any company, internal domains that are not in your DNS, on the Action menu, click New, click Domain, and type the new domains.
The local domain is the one that will be hosted by this SMTP server (and mail will be placed in the Drop directory in the case of IIS 4.0).
A remote domain is the one hosted on a remote SMTP server. Any mail destined for a remote domain that is generated on this SMTP server will be forwarded to the FQDN host in the remote domain field.
If you select the Allow incoming mail to be relayed to this domain option, then mail coming from other SMTP hosts can be relayed also.
Go to SMTP Site Property page and select the Directory Security Property page.
Click the Relay Restrictions button to bring up the Relay Restrictions dialog box.
If you want all computers to be able to relay without any authentication, select the Allowed to Relay option and clear the Allow any computer that successfully authenticates to relay option.
If you want only computers that authenticate to relay, select the Not allowed to relay option and the Allow any computer that successfully authenticates to relay option.
If you do not want any computer, authenticated or not, to be able to relay, select the Not allowed to relay option and clear the Allow any computer that successfully authenticates to relay option.
When you set relay restrictions, you do not have to stop or restart the service; the settings take effect immediately, even for established connections.