Output warning message when using SmtpEmailSender.

framework/src/Volo.Abp.Emailing/Volo/Abp/Emailing/EmailSenderBase.cs

@@ -4,6 +4,8 @@ using System.Linq;
 using System.Net.Mail;
 using System.Text;
 using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 using Volo.Abp.BackgroundJobs;
 
 namespace Volo.Abp.Emailing;
@@ -13,6 +15,8 @@ namespace Volo.Abp.Emailing;
 /// </summary>
 public abstract class EmailSenderBase : IEmailSender
 {
+    public ILogger<EmailSenderBase> Logger { get; set; }
+
     protected IEmailSenderConfiguration Configuration { get; }
 
     protected IBackgroundJobManager BackgroundJobManager { get; }
@@ -22,6 +26,8 @@ public abstract class EmailSenderBase : IEmailSender
     /// </summary>
     protected EmailSenderBase(IEmailSenderConfiguration configuration, IBackgroundJobManager backgroundJobManager)
     {
+        Logger = NullLogger<EmailSenderBase>.Instance;
+
         Configuration = configuration;
         BackgroundJobManager = backgroundJobManager;
     }

framework/src/Volo.Abp.Emailing/Volo/Abp/Emailing/NullEmailSender.cs

@@ -12,15 +12,13 @@ namespace Volo.Abp.Emailing;
 /// </summary>
 public class NullEmailSender : EmailSenderBase
 {
-    public ILogger<NullEmailSender> Logger { get; set; }
-
     /// <summary>
     /// Creates a new <see cref="NullEmailSender"/> object.
     /// </summary>
     public NullEmailSender(IEmailSenderConfiguration configuration, IBackgroundJobManager backgroundJobManager)
         : base(configuration, backgroundJobManager)
     {
-        Logger = NullLogger<NullEmailSender>.Instance;
+
     }
 
     protected override Task SendEmailAsync(MailMessage mail)

framework/src/Volo.Abp.Emailing/Volo/Abp/Emailing/Smtp/SmtpEmailSender.cs

@@ -2,6 +2,7 @@ using System;
 using System.Net;
 using System.Net.Mail;
 using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
 using Volo.Abp.BackgroundJobs;
 using Volo.Abp.DependencyInjection;
 
@@ -71,6 +72,10 @@ public class SmtpEmailSender : EmailSenderBase, ISmtpEmailSender, ITransientDepe
     {
         using (var smtpClient = await BuildClientAsync())
         {
+            Logger.LogWarning("We don't recommend that you use the SmtpClient class for new development because SmtpClient doesn't support many modern protocols. " +
+                               "Use MailKit(https://github.com/jstedfast/MailKit) or other libraries instead." +
+                               "For more information, see https://github.com/dotnet/platform-compat/blob/master/docs/DE0005.md");
+
             await smtpClient.SendMailAsync(mail);
         }
     }