Private pop3Stream As Stream
...
         If m_useSSL Then
          'get SSL stream
          Try
            CallTrace("   Get SSL connection")
            pop3Stream = New SslStream(serverTcpConnection.GetStream(), False)
            pop3Stream.ReadTimeout = m_readTimeout
          Catch ex As Exception
            Throw New Pop3Exception("Server " + m_popServer + " found, but cannot get SSL data stream." & vbLf & "Runtime Error: " + ex.ToString())
          End Try

          'perform SSL authentication
          Try
            CallTrace("   Get SSL authentication")
            DirectCast(pop3Stream, SslStream).AuthenticateAsClient(m_popServer)
          Catch ex As Exception
            Throw New Pop3Exception("Server " + m_popServer + " found, but problem with SSL Authentication." & vbLf & "Runtime Error: " + ex.ToString())
          End Try
        Else

Run Time Error Occured:
Server bla-bla-bla.com found, but problem with SSL Authentication.
Runtime Error: System.Security.Authentication.AuthenticationException: Удаленный сертификат недействителен согласно результатам проверки подлинности.
   в System.Net.Security.SslState.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, Exception exception)
...

WebException: System.Net.WebException: Ошибка подключения к веб-службе: Базовое соединение закрыто:
 Не удалось установить доверительные отношения для защищенного канала SSL/TLS.
 ---> System.Net.WebException: Базовое соединение закрыто: Не удалось установить доверительные отношения для защищенного канала SSL/TLS.
 ---> System.Security.Authentication.AuthenticationException: Удаленный сертификат недействителен согласно результатам проверки подлинности.

        If m_useSSL Then
          'get SSL stream
          Try
            CallTrace("   Get SSL connection")
            ImapStream = New SslStream(serverTcpConnection.GetStream(), False, _
             New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate))
            ImapStream.ReadTimeout = m_readTimeout
          Catch ex As Exception
            Throw New ImapException("Server " + m_ImapServer + " found, but cannot get SSL data stream." & vbCrLf & "Runtime Error: " + ex.ToString())
          End Try

          'perform SSL authentication
          Try
            CallTrace("   Get SSL authentication")
            DirectCast(ImapStream, SslStream).AuthenticateAsClient(m_ImapServer)
          Catch ex As Exception
            Throw New ImapException("Server " + m_ImapServer + " found, but problem with SSL Authentication." & vbCrLf & "Runtime Error: " + ex.ToString())
          End Try
...
    Private Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, _
     ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
      Debug.Print(certificate.Subject)
      If sslPolicyErrors = sslPolicyErrors.None Then
        Return True
      End If

      Debug.Print("Certificate error: {0}", sslPolicyErrors)

      ' Do not allow this client to communicate with unauthenticated servers.
      Return True ' False
    End Function

22.05.2015 14:33:07	myserver.com	   Connect at port 993
22.05.2015 14:33:07	myserver.com	   Get SSL connection
22.05.2015 14:33:07	myserver.com	   Get SSL authentication
22.05.2015 14:33:07	myserver.com	Rx '* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE
 THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION] 
 Courier-IMAP ready. Copyright 1998-2010 Double Precision, Inc.  See COPYING for distribution information.'
22.05.2015 14:33:07	myserver.com	   ImapMailClient Connection State Authorization reached

E=admin@agava.com, OID.2.5.4.41=., CN=cpXXX.agava.net, OU=IT Department, O=AGAVA Software Ltd., L=Moscow, S=Moscow, C=RU
Certificate error: RemoteCertificateNameMismatch, RemoteCertificateChainErrors

Return True

  Private Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, _
     ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
      If sslPolicyErrors = sslPolicyErrors.None Then
        Return True
      End If

      ' Do not allow this client to communicate with unauthenticated servers.
      Return Form1.CheckBoxIgnore.Checked
    End Function

   Private Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, _
     ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean

      If sslPolicyErrors = sslPolicyErrors.None Then
        Return True
      End If

      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateChainErrors) = sslPolicyErrors Then _
        CallWarning("ValidateServerCertificate", "", "The remote certificate is invalid according to the validation procedure.")
      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateNameMismatch) = sslPolicyErrors Then _
        CallWarning("ValidateServerCertificate", "", "The remote certificate common name is not identical to the server name.")
      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateNotAvailable) = sslPolicyErrors Then _
        CallWarning("ValidateServerCertificate", "", "The remote certificate is not available.")

      ' Accept all certificates
      CallWarning("ValidateServerCertificate", "", "Certificate errors ignored.")
      Return True
    End Function

23.05.2015 8:48:50	server.com	   Connect at port 993
23.05.2015 8:48:50	server.com	   Get SSL connection
23.05.2015 8:48:50	server.com	   Get SSL authentication
23.05.2015 8:48:50	server.com	!! The remote certificate is invalid according to the validation procedure.
23.05.2015 8:48:50	server.com	!! The remote certificate common name is not identical to the server name.
23.05.2015 8:48:50	server.com	!! Certificate errors ignored.
23.05.2015 8:48:50	server.com	Rx '* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE 
THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION]
 Courier-IMAP ready. Copyright 1998-2010 Double Precision, Inc.  See COPYING for distribution information.'

      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateChainErrors) = sslPolicyErrors Then
        CallWarning("ValidateServerCertificate", "", "The remote certificate is invalid according to the validation procedure.")
        If chain.ChainStatus.Length <> 0 Then
          For Each objChainStatus As X509ChainStatus In chain.ChainStatus
            CallTrace("   " + objChainStatus.Status.ToString() + " - " + Replace(objChainStatus.StatusInformation, vbCrLf, ""))
          Next
        End If
      End If
      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateNameMismatch) = sslPolicyErrors Then _
        CallWarning("ValidateServerCertificate", "", "The remote certificate common name is not identical to the server name.")
      If (sslPolicyErrors Or sslPolicyErrors.RemoteCertificateNotAvailable) = sslPolicyErrors Then _
        CallWarning("ValidateServerCertificate", "", "The remote certificate is not available.")

      ' Accept all certificates
      CallWarning("ValidateServerCertificate", "", "Certificate errors ignored.")
      Return True

23.05.2015 9:55:13	server.com	   Connect at port 993
23.05.2015 9:55:13	server.com	   Get SSL connection
23.05.2015 9:55:13	server.com	   Get SSL authentication
23.05.2015 9:55:13	server.com	!! The remote certificate is invalid according to the validation procedure.
23.05.2015 9:55:13	server.com	   PartialChain - Не удается построить цепочку сертификатов для доверенного корневого центра.
23.05.2015 9:55:13	server.com	!! The remote certificate common name is not identical to the server name.
23.05.2015 9:55:13	server.com	!! Certificate errors ignored.
23.05.2015 9:55:13	server.com	Rx '* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE 

    Try
      Dim client As New SmtpClient("server.com", 25) '587
       client.Credentials = New System.Net.NetworkCredential("user@server.com", "password")
      client.EnableSsl = True

      Dim message As New MailMessage(New MailAddress("user@server.com"), New MailAddress("friend@gmail.com"))
      message.Body = "Test body"
      message.Subject = "Test subject"
       ServicePointManager.ServerCertificateValidationCallback = _
       New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate) 'включаем коллбэк 
      client.Send(message)
      MsgBox("OK")
    Catch
      MsgBox(Err.Description)
    End Try
    ServicePointManager.ServerCertificateValidationCallback = Nothing 'отключаем коллбэк

  Private Function ValidateServerCertificate(ByVal sender As Object, ByVal certificate As X509Certificate, _
   ByVal chain As X509Chain, ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
    ' Accept all certificates
    Return True
  End Function

    ServicePointManager.ServerCertificateValidationCallback = _
     New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate)
    Try
      Dim o_Mess As Object = CreateObject("CDO.Message")
      Dim v_Conf As String = "http://schemas.microsoft.com/cdo/configuration/"
      With o_Mess
        '===(В-1)код необходим для добавления utf-8 в Subject(если .AddAttachment стоит до .TextBodyPart.Charset)===
        .BodyPart.Charset = "utf-8"
        '===(В-1)код необходим для добавления utf-8 в Subject(если .AddAttachment стоит до .TextBodyPart.Charset)===
        .To = "Получатель<friend@gmail.com"
        .From = "Отправитель<user@server.com>;"
        .Subject = "Тема сообщения"
        .TextBody = "Текст сообщения"
        .TextBodyPart.Charset = "utf-8"
        With .Configuration.Fields
          .item(v_Conf & "sendusing") = 2
          .item(v_Conf & "smtpserver") = "server.com"
          .item(v_Conf & "smtpserverport") = 465 '25 либо 465
          .item(v_Conf & "smtpauthenticate") = 1 '1-использовать
          .item(v_Conf & "sendusername") = "user@server.com"
          .item(v_Conf & "sendpassword") = "password"
          .item(v_Conf & "smtpusessl") = True 'использовать SSL
          .item(v_Conf & "smtpconnectiontimeout") = 60 'пока считаем что 60, потом м.б. добавим параметр
          .Update()
        End With
        .Send()
      End With
      MsgBox("Test mail was sent successfully!", vbInformation + vbOKOnly, "Email was sent")
    Catch
      MsgBox("Sending mail...Failed." & vbCrLf & "Error code: " & Err.Number & vbCrLf & Err.Description, _
       vbCritical + vbOKOnly, "Sending mail failed.")
    End Try
    ServicePointManager.ServerCertificateValidationCallback = Nothing

-2147220973 Транспорту не удалось подключиться к серверу.

ServicePointManager.ServerCertificateValidationCallback = _
     New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate)

ServicePointManager.ServerCertificateValidationCallback = Nothing 'отключаем коллбэк

          'Активируем CallBack отключения проверки сертификата SSL сервера если еще не активирован (при необходимости)
          If m_smtp_UseSSL Then
            If System.Net.ServicePointManager.ServerCertificateValidationCallback Is Nothing Then _
             System.Net.ServicePointManager.ServerCertificateValidationCallback = _
             New RemoteCertificateValidationCallback(AddressOf ValidateServerCertificate)
          End If

  Public Const WINHTTP_ACCESS_TYPE_DEFAULT_PROXY = 0
  Public Const WINHTTP_NO_PROXY_NAME = 0
  Public Const WINHTTP_NO_PROXY_BYPASS = 0

  Public Declare Function WinHttpCloseHandle Lib "winhttp.dll" _
   (ByVal hInternet As IntPtr) As Boolean
  Public Declare Function WinHttpOpen Lib "winhttp.dll" _
   (ByVal pwszUserAgent As String, ByVal dwAccessType As Integer, ByVal pwszProxyName As String, _
   ByVal pwszProxyBypass As String, ByVal dwFlags As Integer) As IntPtr

  Public Delegate Sub WINHTTP_STATUS_CALLBACK_Delegate(ByVal hInternet As IntPtr, _
    ByVal dwContext As IntPtr, ByVal dwInternetStatus As IntPtr, _
    ByVal lpvStatusInformation As IntPtr, ByVal dwStatusInformationLength As Integer)

  Public Const WINHTTP_CALLBACK_FLAG_ALL_NOTIFICATIONS As UInteger = &HFFFFFFFFUI

  Public Declare Function WinHttpSetStatusCallback Lib "winhttp.dll" _
   (ByVal hInternet As IntPtr, ByVal lpfnInternetCallback As WINHTTP_STATUS_CALLBACK_Delegate, _
   ByVal dwNotificationFlags As UInteger, ByVal dwReserved As IntPtr) As WINHTTP_STATUS_CALLBACK_Delegate

  Public Declare Function WinHttpConnect Lib "winhttp.dll" _
   (ByVal hSession As IntPtr, ByVal pswzServerName As String, ByVal nServerPort As Integer, _
   ByVal dwReserved As Integer) As IntPtr

  Dim hSession As IntPtr

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    hSession = WinHttpOpen("A WinHTTP Example Program/1.0",
                                    WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                    WINHTTP_NO_PROXY_NAME,
                                    WINHTTP_NO_PROXY_BYPASS, 0)
    If (hSession) Then
      ' Install the status callback function.
      WinHttpSetStatusCallback(hSession, _
       AddressOf WINHTTP_STATUS_CALLBACK, WINHTTP_CALLBACK_FLAG_ALL_NOTIFICATIONS, IntPtr.Zero)

      'Place additional code here.
      WinHttpConnect(hSession, "server.com", 993, 0)

      ''When finished, release the HINTERNET handle.
      'WinHttpCloseHandle(hSession)
    Else
      Debug.Print("Error in WinHttpOpen.")
    End If

  End Sub

  Private Sub ButtonConnect_Click(sender As Object, e As EventArgs) Handles ButtonConnect.Click
    Dim hSession_l As IntPtr = WinHttpOpen("A WinHTTP Example Program/1.0",
                                    WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                    WINHTTP_NO_PROXY_NAME,
                                    WINHTTP_NO_PROXY_BYPASS, 0)
    WinHttpConnect(hSession_l, "server.com", 993, 0)

  End Sub

  Public Sub WINHTTP_STATUS_CALLBACK(ByVal hInternet As IntPtr, _
    ByVal dwContext As IntPtr, ByVal dwInternetStatus As IntPtr, _
    ByVal lpvStatusInformation As IntPtr, ByVal dwStatusInformationLength As Integer)
    Debug.Print("WINHTTP_STATUS_CALLBACK")
  End Sub