Sending mail using SmtpClient in .net

Question

I am unable to send the mail using smtp client. here is the code:

SmtpClient client=new SmtpClient("Host");
client.Credentials=new NetworkCredential("username", "password");
MailMessage mailMessage = new MailMessage();
mailMessage.from="[email protected]";
mailMessage.To.Add("[email protected]");
mailMessage.body="body";
mailMessage.subject="subject";
client.Send(mailMessage);

The problem is that when I use this code in ASP.NET application, I do not receive any mails. When in asp.net I change the from mail address to username given in NetworkCredential, I receive mails.

But in C# windows application, I can get emails, even if sender's email address is not valid.

Answer 1

I figured out that setting the SmtpClient Credentials property before setting the ' UseDefaultCredentials = false ' UseDefaultCredentials = false causes the credentials to be ignored.

Fails:

SmtpClient smtp = new SmtpClient;
smtp.Credentials = new NetworkCredential("user","pass");
smtp.UseDefaultCredentials = false;

Works:

SmtpClient smtp = new SmtpClient;
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential("user","pass");

Go figure.

** UPDATE **

The reason the order is important here is that the setter on the UseDefaultCredentials property actually sets the Credentials to null via the decompiled code:

/// <summary>Gets or sets a <see cref="T:System.Boolean" /> value that controls whether the <see cref="P:System.Net.CredentialCache.DefaultCredentials" /> are sent with requests.</summary>
/// <returns>
/// <see langword="true" /> if the default credentials are used; otherwise <see langword="false" />. The default value is <see langword="false" />.</returns>
/// <exception cref="T:System.InvalidOperationException">You cannot change the value of this property when an e-mail is being sent.</exception>
public bool UseDefaultCredentials
{
  get
  {
    return this.transport.Credentials is SystemNetworkCredential;
  }
  set
  {
    if (this.InCall)
      throw new InvalidOperationException(SR.GetString("SmtpInvalidOperationDuringSend"));
    this.transport.Credentials = value ? (ICredentialsByHost) CredentialCache.DefaultNetworkCredentials : (ICredentialsByHost) null;
  }
}

Answer 2

The short answer : Do not use .net the internal SMTP client class - use MailKit.

The long answer :

1. It is marked as obsolete in MS docs

[System.Obsolete("SmtpClient and its network of types are poorly designed, we strongly recommend you use https://github.com/jstedfast/MailKit and https://github.com/jstedfast/MimeKit instead")] public class SmtpClient : IDisposable

1. Use the oss lib MailKit.
2. Elaboration : Because you will face its deficiencies sooner rather than later. Read here or read around the internet.
3. Here is an example for sending plain messages and multipart messages.

I intentionally did not copy-paste the examples from the docs here because the documentation may change over time and it is better to read .