I’m confused about why you say the problem is related to .net standard. The documentation of SmtpPickupDirAppender says

/// The configuration for this appender is identical to that of the <c>SMTPAppender</c>, /// except that instead of specifying the <c>SMTPAppender.SMTPHost</c> you specify  /// <see cref="PickupDir"/>. 

This appender does not work with a Smtp host but with a file system directory. You need to set PickupDir property.

Change SmtpPickupDirAppender by SmtpAppender in case you want to send emails for real or replace

smtpElement.SmtpHost = FX_CONNECT.EmailElement.Server; 

by

smtpElement.PickupDir = "C:\YourPickUpDir"; 

in case you want to keep using that appender.

Thank you @Claudio Redi for your comment, It gave me the right direction to solve the issue.

I added the below class to my project and used the SMTPAppender class in place of log4net.Appender.SmtpPickupDirAppender in the line var smtpElement = (SMTPAppender)AppLogger.Logger.Repository.GetAppenders().Where(appender => appender.Name.Equals(element.Name)).FirstOrDefault(); and it’s working as expected.

using log4net.Appender; using log4net.Core; using System.IO; using System.Net.Mail;  namespace FxCore.Diagnostics.Components {     public class SMTPAppender : BufferingAppenderSkeleton     {         public string To { get; set; }         public string From { get; set; }         public string Subject { get; set; }         public string SmtpHost { get; set; }         public string Port { get; set; }          protected void SendEmail(string messageBody)         {             SmtpClient client = new SmtpClient(SmtpHost);             client.UseDefaultCredentials = false;             client.Port = int.Parse(Port);             using (MailMessage mailMessage = new MailMessage())             {                 mailMessage.From = new MailAddress(From);                 mailMessage.To.Add(To);                 mailMessage.Body = messageBody;                 mailMessage.Subject = Subject;                 client.Send(mailMessage);             }         }          protected override bool RequiresLayout => true;          protected override void SendBuffer(LoggingEvent[] events)         {             StringWriter writer = new StringWriter(System.Globalization.CultureInfo.InvariantCulture);              string t = Layout.Header;             if (t != null)             {                 writer.Write(t);             }              for (int i = 0; i < events.Length; i++)             {                 // Render the event and append the text to the buffer                 RenderLoggingEvent(writer, events[i]);             }              t = Layout.Footer;             if (t != null)             {                 writer.Write(t);             }              SendEmail(writer.ToString());         }     } }