Question

I want to Run Below code without

NetworkCredential nc = new Net.NetworkCredential("USERNAME", "PASSWORD"). 

BY using Only Exchange Host (Server Name) And Port

Im Getting Error For this code : Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender

protected void SendEmail(object sender, EventArgs e)
{
    SmtpClient smtpClient = new SmtpClient("ExchangeServerName",25);
    MailMessage message = new MailMessage();
    try
    {
        MailAddress fromAddress = new MailAddress("[e-mail]", "From Me");
        MailAddress toAddress = new MailAddress("[e-mail]", "To You");
        message.From = fromAddress;
        message.To.Add(toAddress);
        message.Subject = "Testing!";
        message.Body = "This is the body of a sample message";
        smtpClient.UseDefaultCredentials = true;
        System.Net.NetworkCredential nc = CredentialCache.DefaultNetworkCredentials;
        smtpClient.Credentials = (System.Net.ICredentialsByHost)nc.GetCredential("ExchangeServerName", 25, "Basic");
        smtpClient.Send(message);
        lblText.Text ="Email sent.";
    }
    catch (Exception ex)
    {
        lblText.Text = "Coudn't send the message!\n  " + ex.Message;
    }
}

Answer 1

I have done it. For more details about my code use this link.

Below Code is worked Fine with

Server : Windows Server 2003,Windows Server 2008,Windows Server 2008 R2

IIS : 6.0, 7.0

.Net Frame Wotk : 2.0,3.5,4.0

string sMessage;
SmtpClient smtpClient = new SmtpClient();
MailMessage message = new MailMessage();
try
{

 //you can provide invalid from address. but to address Should be valil
MailAddress fromAddress = new MailAddress("[e-mail]", "BALA");

smtpClient.Host = "Exchange Server Name";
smtpClient.Port = 25;
//smtpClient.Port = 587;


smtpClient.UseDefaultCredentials = true; 

message.From = fromAddress;
message.To.Add([e-mail]); //Recipent email 
message.Subject = _subject;
message.Body = _details;
message.IsBodyHtml = true;

//smtpClient.EnableSsl = true; 

smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;

smtpClient.Send(message); 

sMessage = "Email sent.";
}
catch (Exception ex)
{
sMessage = "Coudn't send the message!\n " + ex.Message;
}


lblMailStatus.Text = sMessage;