SMTP Failed Recipient Exception - C#

In this Exception and Solution Series, today the solution to a System.Net.Mail.SmtpFailedRecipientException I got recently trying to send an email from a Console Application using the Smtp class from System.Net.Mail library.

Exception

System.Net.Mail.SmtpFailedRecipientException HResult=0x80131500 Message=Mailbox unavailable. The server response was: 5.7.1 Unable to relay Source=System StackTrace: at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at <<NameOfMyConsoleApplication>>.Program.Main(String[] args) in C:\<<PathToMySolution>>\Program.cs:line 52

Solution

I just added the following system.net configuration to my App.Config file under the <configuration> tag

<system.net> <mailSettings> <smtp deliveryMethod="Network"> <network host="mail" defaultCredentials="true"/> </smtp> </mailSettings> </system.net>

That resolved the Exception. Thank you Vijaya Malla Vijaya Malla