Smdn.Net.Imap4

using System;
using System.Net;

using Smdn.Net.Imap4.WebClients;

class GimapFetchExample {
  public static void Main()
  {
    ImapWebRequestCreator.RegisterPrefix();
    ImapSessionManager.ServerCertificateValidationCallback += delegate {
      return true;
    };

    using (var client = new WebClient()) {
      client.Credentials = new NetworkCredential("username", "password");
      client.DownloadFile("imaps://username@imap.gmail.com/INBOX/;UID=1", "message.eml");
    }
  }
}

Imports System
Imports System.Net
Imports System.Net.Security
Imports System.Security.Cryptography.X509Certificates

Imports Smdn.Net.Imap4.WebClients

Class GimapFetchExample
  Public Shared Sub Main()
    ImapWebRequestCreator.RegisterPrefix()
    ImapSessionManager.ServerCertificateValidationCallback = New RemoteCertificateValidationCallback(AddressOf ValidateRemoteCertificate)

    Using client As New WebClient()
      client.Credentials = New NetworkCredential("username", "password")
      client.DownloadFile("imaps://username@imap.gmail.com/INBOX/;UID=1", "message.eml")
    End Using
  End Sub

  Private Shared Function ValidateRemoteCertificate(ByVal sender As Object, _
                                                    ByVal certificate As X509Certificate, _
                                                    ByVal chain As X509Chain, _
                                                    ByVal sslPolicyErrors As SslPolicyErrors) As Boolean
    Return True
  End Function
End Class