.NET SMTP Client - Client does not have permission

Question

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.

try
{
    var client = new SmtpClient("MailServer");
    client.UseDefaultCredentials = true;

    MailMessage message = new MailMessage("[e-mail]", "[e-mail]", "Test Message", "Test Body");
    client.Send(message);
}
catch (Exception ex)
{
    // Do Nothing
}

I have deployed on three environments; on Windows 7 (using VS 2010 IIS) it sends the email fine, on the Windows 2003 IIS6 machine it sends the email fine, finally on the Windows 2008 R2 II7 server I get the following error:

Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender using username 

Can anybody advise on what may be causing this. I have noticed that when I view User.Identity.Name, this is returning an empty string.

Answer 1

If you are using an exchange server and the logon account and sender email are different you will got the error "does not have permissions to send...". This is because of the account permissions. You must grant "send as" permission to the logon account.

Answer 2

For me it was using different credentials -

In network credentials I was using -

new NetworkCredential(smtpUser, smtpPassword)

and in fromAddress

 var fromAddress = new MailAddress("[e-mail]", string.Empty);

Where when sending email where fromAddress is different than actual network credentials will lead this issue. See below -

 using (var message = new System.Net.Mail.MailMessage(fromAddress, toAddress)
                {
                    Subject = Keys.MailSubject,
                    Body = body,
                    IsBodyHtml = true
                })
                {
                    smtp.Send(message);

                }

The simple fix is to keep both the mails same as network credentials's one.