Outlook MailItem embed images not shown in sent mails

I have the following code to embed images to Outlook MailItems:

private void ReplaceImageIds()
{
   foreach(var image in Image.GetImagesFromText(HTMLBody))
   {
      var imageTag = $"<img src \" cid:{image.Id.ToString()} \"/>";

      var attachment = _mailItem.Attachments.Add(image.FilePath, OlAttachmentType.olEmbeddeditem, null, "");
      attachment.PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/proptag/0x370E001F", "image/png");
      attachment.PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/proptag/0x3712001F", image.Id.ToString());

      HTMLBody = HTMLBody.Replace($"ImageId={image.Id.ToString()}", imageTag);
   }
}

That works just fine. The images are shown when i receive the e-mail - but just there.

When i take a look at my sent-mails folder in outlook the images are just shown like this:

Does anyone have an idea why they are shown like this and can help me to fix that?

I'm confused about that because the images are shown when i receive the e-mail.

Mail is send like this:

public Boolean Send()
{
   // Check if all properties are set.
   Validate();

   try
   {
      var oApp = new Application();

      var oNS = oApp.GetNamespace("mapi");

      oNS.Logon();

      _mailItem = oApp.CreateItem(OlItemType.olMailItem) as MailItem;

      // Set To, CC and BCC.
      AddRecipients();

      // Replace images.
      ReplaceImageIds();

      if (Body != null)
         _mailItem.Body = Body;

      if(HTMLBody != null)
         _mailItem.HTMLBody = HTMLBody;              

      _mailItem.Subject = Subject;

      // Set account to send.
      SetSendingAccount(oApp);

      // Add attachments.
      AddAttachments();

      _mailItem.Send();

      oNS.Logoff();

      return true;
   }
   catch (System.Exception ex)
   {
      Utils.LogException(ex, "Could not send email.");
      throw new System.Exception("Could not send email.", ex);
   }
}

Thanks in advance.