client.Connect("imap.exmail.qq.com", 993, SecureSocketOptions.SslOnConnect);

client.Connect("imap.exmail.qq.com", 143, SecureSocketOptions.None);

client.Authenticate(MAIL_NAME, MAIL_PASSWORD);

client.Inbox.Open(FolderAccess.ReadWrite);

search for messages where the Subject header contains either "MimeKit" or "MailKit"
  var query = SearchQuery.SubjectContains("MimeKit").Or(SearchQuery.SubjectContains("MailKit"));
  var uids = client.Inbox.Search(query);

var uids = client.Inbox.Search(SearchQuery.All);

string subject = message.Subject;
  if (!subject.Contains("MimeKitDemo"))
    return;

string body = message.TextBody ?? string.Empty;
  if (!body.Contains("MimeKitDemoBody"))
    return;

var attachments = message.Attachments;
  if (attachments.Any())
  {
    foreach (var attachment in attachments)
      DownloadAttachment(attachment);
  }

  private static void DownloadAttachment(MimeEntity attachment)
  {
    if (attachment is MessagePart)
    {
      var fileName = attachment.ContentDisposition?.FileName;
      var rfc822 = (MessagePart)attachment;

      if (string.IsNullOrEmpty(fileName))
        fileName = "attached-message.eml";

      var path = Path.Combine(DIRECTORY, fileName);
      using (var stream = File.Create(path))
        rfc822.Message.WriteTo(stream);
    }
    else
    {
      var part = (MimePart)attachment;
      var fileName = part.FileName;

      var path = Path.Combine(DIRECTORY, fileName);
      using (var stream = File.Create(path))
        part.Content.DecodeTo(stream);
    }
  }

client.Inbox.MoveTo(uid, client.GetFolder(SpecialFolder.Trash));

client.Inbox.AddFlags(uid, MessageFlags.Deleted, true);
  client.Inbox.Expunge();