SMTP Exception: 550 5.7.1 Unable to Relay

I was trying to configure IIS 6 SMTP server for outbound mail in Windows Server 2008. I was getting an error highlighted below:

Telnet: 550 5.7.1 Unable to relay for ferdous@congral.com

I am sharing my experience here on how to solve this error. This is an error reply from your SMTP mail server. It indicates that your mail server is not configured to allow you to send mail through it. The standard port is 25 for SMTP connections. You can check your SMTP Server on SMTP port 25 using Telnet command. Open "MS-DOS prompt" or "command prompt". Just type the following command in the command prompt window and press "Enter":

telnet domain-name.com 25

The above highlighted line shows the error. So, you need to do the following if you get the SMTP error message "550 5.7.1 Unable to relay".

By default, the SMTP virtual server allows relaying only from authenticated users. This configuration is designed to prevent unauthorized users from using your Exchange server to relay mail. The virtual server's default configuration allows only authenticated computers to relay mail.

Change Default Relay Restriction

Go to IIS6.0 Manager –> SMTP Virtual Server Properties –> Go to Access tab

Change to "All except the list below" to remove restriction. You also add Restricted IPs or domain name that you don’t want to allow.

This should solve your problem to allow to send email anywhere like gmail, yahoo or any other domain.