How do I send emails outside my domain with Exchange 2007 and c#

Question

I am able to send emails using the typical C# SMTP code across Exchange 2007 as long as both the from and to addresses are within my domain.

As soon as I try to send emails outside the domain I get:

Exception Details: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay

How can I get exchange to accept my email and send it out to the internet?

Answer 1

Try #2... How about using a Exchange Pickup Folder instead? They are a faster way to send emails through Exchange because it just creates the email and drops it in the folder, no waiting to connect to the server or waiting for a reply. Plus I think it skips the whole relay issue.

Configure youur SmtpClient like so:

SmtpClient srv = new SmtpClient("exchsrv2007", 25) {
    DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory,
    PickupDirectoryLocation = "\\exchsrv2007\PickupFolder"
}
...

Answer 2

Authenticate to the exchange server.

http://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.credentials.aspx

---

DefaultNetworkCredentials returns empty strings for username etc and causes this exception...

Here is an example, and here is another of sending authenticated message with System.Net.Mail.