Error sending email to external accounts

I am trying to setup a Forgot Password page that will email the user a temporary password. If I set the to email to an internal address, it works, but if it is set to any external address it does not.

I get a SmtpFailedRecipientException with a message of "Mailbox unavailable. The server response was: <[email protected]> No such user here".

He is my code:

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Mail;
using SC.TAS.OKTLE.Common.BaseClasses;
using SC.TAS.OKTLE.Common.Interfaces;
using SC.TAS.OKTLE.Common.Interfaces.Email;
using SC.TAS.OKTLE.Common.RequestResponseObjects;

namespace SC.TAS.OKTLE.Common.Utility
{

public class Emailer : IEmailer
{

private readonly SettingsReaderBase _settingsReader;
private readonly ILogger _logger;

public Emailer()
{
_settingsReader = DependencyResolver.SettingsReader;
_logger = DependencyResolver.Logger;
}

#region IEmailer Members

public ServiceResponse Send(string to, string subject, string body)
{
ServiceResponse response = new ServiceResponse();
SmtpClient mailClient = new SmtpClient();
try
{
NetworkCredential networkCredential = new NetworkCredential("[email protected]", "********", "oktledev.com");
mailClient.DeliveryMethod = SmtpDeliveryMethod.Network;
mailClient.EnableSsl = false;
mailClient.Host = "mail.oktledev.com";
mailClient.UseDefaultCredentials = false;
mailClient.Credentials = networkCredential;

mailClient.Send("[email protected]", "[email protected]", subject, body);
}
catch (SmtpException smtpException)
{
_logger.LogError(smtpException);
response.ApplicationErrors = new List<string> { "There was an error sending the email." };
return response;
}
return response;
}

#endregion
}
}

Please advise as I have tried suggestions in other post and have not found an answer.

Thank you,

Michael Wheeler