Mailbox unavailable. The server response was: 5.4.1 Relay Access Denied

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C#3.5

.NET4

SMTP

I am getting "Mailbox unavailable. The server response was: 5.4.1 Unable to relay for myemail@mydomain.com" when I try to send the mail using C# code.

Then I tested my smtp server at https://www.wormly.com/test_smtp_server.

Here when I enter my smtp server, sender and recipient name. I got following message:

MAIL FROM: mymail@mydomain.com

SMTP -> FROM SERVER:

250 2.1.0 Sender OK

RCPT TO: anothermail@mydomain.com

SMTP -> FROM SERVER:

550 5.4.1 Relay Access Denied

SMTP -> ERROR: RCPT not accepted from server: 550 5.4.1 Relay Access Denied

Message sending failed.