Unable to send to all recipients … Mailbox unavailable. The server response was: 5.7.1 Unable to relay for …

Managed to fix it by going to IIS > Default SMTP Virtual Server > Property > Access > Relay Even though I still not able to receive the mail I sent out yet, but there is no more error message when I use VB.NET code (System.Net.Mail.MailMessage) to send it. When I went to C:InetpubmailrootQueue, I Read more about Unable to send to all recipients … Mailbox unavailable. The server response was: 5.7.1 Unable to relay for …[…]