I'm getting strange occurance on our servers when I am trying to send an email using SmtpClient class via an ASP MVC3 project. This is the code I am using.
var client = new SmtpClient("MailServer");
client.UseDefaultCredentials = true;
MailMessage message = new MailMessage("firstname.lastname@example.org", "email@example.com", "Test Message", "Test Body");
catch (Exception ex)
// Do Nothing
Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender using username
It is likely that the mail server does not support sending anonymous emails and will require credentials to be specified which are registered on the mail server.
You can specify the credentials like so:
client.UseDefaultCredentials = false; client.Credentials = new System.Net.NetworkCredential("username", "password");
Hope this helps.