When you want log commands sent to the server and the corresponding responses, the following events are very useful for diagnostic purposes:

The steps below show you how to handle the CommandResponse event to log commands sent to the server and responses:

Using CommandResponse event to make a trace log

1. Add using directives to your code to create aliases for existing namespaces and avoid having to type the fully qualified type names. The code looks similar to the following:
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   C#

   VB.NET

   using System;
   using System.Text;
   using ComponentPro.Net;
   using ComponentPro.Net.Mail;
   

   Imports System.Text
   Imports ComponentPro.Net
   Imports ComponentPro.Net.Mail
   

2. Create a new instance of the Pop3 class.
3. Register handler to the CommandResponse event.
4. Now you can connect to the POP3 server with Connect methods. The code looks similar to the following:
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   C#

   VB.NET

   // Create a new instance.
   Pop3 client = new Pop3();
   
   // Register event handlers.
   client.CommandResponse += client_CommandResponse;
   
   // Connect to the POP3 server.
   client.Connect("server");
   

   ' Create a new instance. 
   Dim client As New Pop3()
   
   ' Register event handlers. 
   AddHandler client.CommandResponse, AddressOf client_CommandResponse
   
   ' Connect to the POP3 server.
   client.Connect("server")
   

5. Use your user name and password to login with Authenticate methods. The code looks similar to the following:
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   C#

   VB.NET

   // Authenticate.
   client.Authenticate("test", "test");
   

   ' Authenticate.
   client.Authenticate("test", "test")
   

6. Do your work like listing messages, downloading mail messages, etc.  The code looks similar to the following:
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   C#

   VB.NET

   StringBuilder sb = new StringBuilder();
   
   Pop3MessageCollection list = client.ListMessages(Pop3EnvelopeParts.Size | Pop3EnvelopeParts.UniqueId);
   for (int i = 0; i < list.Count; i++)
   {
       sb.AppendFormat("{0} - {1}\r\n", i + 1, list[i].UniqueId);
   }
   
   Console.WriteLine(sb.ToString());
   

   Dim sb As New StringBuilder()
   
   Dim list As Pop3MessageCollection = client.ListMessages(Pop3EnvelopeParts.Size Or Pop3EnvelopeParts.UniqueId)
   For i As Integer = 0 To list.Count - 1
       sb.AppendFormat("{0} - {1}" & vbCrLf, i + 1, list(i).UniqueId)
   Next i
   
   Console.WriteLine(sb.ToString())
   

7. After completing your work, call the Disconnect method to close the POP3 session. 

Final example code

using System;
using System.Text;
using ComponentPro.Net;
using ComponentPro.Net.Mail;

...

public void HandleCommandSentResponseReadEvents()
{
    // Create a new instance. 
    Pop3 client = new Pop3();

    // Register event handlers. 
    client.CommandResponse += client_CommandResponse;

    // Connect to the POP3 server. 
    client.Connect("server");

    // Authenticate. 
    client.Authenticate("test", "test");

    // ... 
 
    // Do something here 
    // ... 
 
    StringBuilder sb = new StringBuilder();

    Pop3MessageCollection list = client.ListMessages(Pop3EnvelopeParts.Size | Pop3EnvelopeParts.UniqueId);
    for (int i = 0; i < list.Count; i++)
    {
        sb.AppendFormat("{0} - {1}\r\n", i + 1, list[i].UniqueId);
    }

    Console.WriteLine(sb.ToString());

    // ... 
 
    // Disconnect. 
    client.Disconnect();
}

void client_CommandResponse(object sender, CommandResponseEventArgs e)
{
    if (e.Command != null)
        Console.WriteLine("CMD>       " + e.Command);
    else 
        Console.WriteLine("RESPONSE>  " + e.Response);
}

Imports System.Text
Imports ComponentPro.Net
Imports ComponentPro.Net.Mail

...

Public Sub HandleCommandSentResponseReadEvents()
    ' Create a new instance. 
    Dim client As New Pop3()

    ' Register event handlers. 
    AddHandler client.CommandResponse, AddressOf client_CommandResponse

    ' Connect to the POP3 server. 
    client.Connect("server")

    ' Authenticate. 
    client.Authenticate("test", "test")

    ' ... 
 
    ' Do something here 
    ' ... 
 
    Dim sb As New StringBuilder()

    Dim list As Pop3MessageCollection = client.ListMessages(Pop3EnvelopeParts.Size Or Pop3EnvelopeParts.UniqueId)
    For i As Integer = 0 To list.Count - 1
        sb.AppendFormat("{0} - {1}" & vbCrLf, i + 1, list(i).UniqueId)
    Next i

    Console.WriteLine(sb.ToString())

    ' ... 
 
    ' Disconnect. 
    client.Disconnect()
End Sub 
 
Private Sub client_CommandResponse(ByVal sender As Object, ByVal e As CommandResponseEventArgs)
    If e.Command IsNot Nothing Then 
        Console.WriteLine("CMD>       " & e.Command)
    Else 
        Console.WriteLine("RESPONSE>  " & e.Response)
    End If 
End Sub