Question

I want to be able to specify multiple smtp server host addresses and implement a logic whereby if email using one smtp server fails, it tries to send using the next smtp server address. Is it possible using log4net. Can we override some functions of log4net and implement our own logic in it to send emails?

<appender name="SmtpAppender" type="log4net.Appender.SmtpAppender">
  <to value="[e-mail]" />
  <from value="[e-mail]" />
  <subject>Error Notification</subject>
  **<smtpHost value="10.0.0.12, 10.0.0.13" />**
  <authentication value="None" />
  <port value="25" />
  <bufferSize value="1" />
  <EnableSsl value="false"/>
  <lossy value="true" />
  <evaluator type="log4net.Core.LevelEvaluator">
    <threshold value="ERROR"/>
  </evaluator>
  <layout type="log4net.Layout.PatternLayout">
    <conversionPattern value="%newline%date [%thread] %level %logger - %message%newline%newline%exception" />
  </layout>
</appender>

Answer 1

It is possible only if you implement custom SmtpAppender.

I have copied CustomSmtpAppender code from Log4net sourcecode Examples. hopefullly this will help you.

don’t forget to refer CustomSmtpAppender in your app config

using System;
using System.IO;
using System.Web.Mail;

using log4net.Layout;
using log4net.Core;
using log4net.Appender;

namespace SampleAppendersApp.Appender
{
    /// <summary>
    /// Simple mail appender that sends individual messages
    /// </summary>
    /// <remarks>
    /// This SimpleSmtpAppender sends each LoggingEvent received as a
    /// separate mail message.
    /// The mail subject line can be specified using a pattern layout.
    /// </remarks>
    public class SimpleSmtpAppender : AppenderSkeleton
    {
        public SimpleSmtpAppender()
        {   
        }

        public string To 
        {
            get { return m_to; }
            set { m_to = value; }
        }

        public string From 
        {
            get { return m_from; }
            set { m_from = value; }
        }

        public PatternLayout Subject 
        {
            get { return m_subjectLayout; }
            set { m_subjectLayout = value; }
        }

        public string SmtpHost
        {
            get { return m_smtpHost; }
            set { m_smtpHost = value; }
        }

        #region Override implementation of AppenderSkeleton

        override protected void Append(LoggingEvent loggingEvent) 
        {
            try 
            {     
                StringWriter writer = new StringWriter(System.Globalization.CultureInfo.InvariantCulture);

                string t = Layout.Header;
                if (t != null)
                {
                    writer.Write(t);
                }

                // Render the event and append the text to the buffer
                RenderLoggingEvent(writer, loggingEvent);

                t = Layout.Footer;
                if (t != null)
                {
                    writer.Write(t);
                }

                MailMessage mailMessage = new MailMessage();
                mailMessage.Body = writer.ToString();
                mailMessage.From = m_from;
                mailMessage.To = m_to;

                if (m_subjectLayout == null)
                {
                    mailMessage.Subject = "Missing Subject Layout";
                }
                else
                {
                    StringWriter subjectWriter = new StringWriter(System.Globalization.CultureInfo.InvariantCulture);
                    m_subjectLayout.Format(subjectWriter, loggingEvent);
                    mailMessage.Subject = subjectWriter.ToString();
                }

                if (m_smtpHost != null && m_smtpHost.Length > 0)
                {
                    SmtpMail.SmtpServer = m_smtpHost;
                }

                SmtpMail.Send(mailMessage);
            } 
            catch(Exception e) 
            {
                ErrorHandler.Error("Error occurred while sending e-mail notification.", e);
            }       
        }

        override protected bool RequiresLayout
        {
            get { return true; }
        }

        #endregion // Override implementation of AppenderSkeleton

        private string m_to;
        private string m_from;
        private PatternLayout m_subjectLayout;
        private string m_smtpHost;
    }
}