Is Outlook changing body format after attaching to ApplicationEvents_11_ItemSendEventHandler and why if so?

I have noticed that the email formatting was changed when I do this:

private void button1_Click(object sender, RibbonControlEventArgs e)
{
  //var mail = new MailForm();
  //mail.ShowDialog();

  Outlook.Application outlookApp = Globals.ThisAddIn.Application;
  outlookApp.ItemSend += new Outlook.ApplicationEvents_11_ItemSendEventHandler(OutlookApp_ItemSend);

  try
  {
    Outlook.MailItem mailItem = (Outlook.MailItem)outlookApp.CreateItem(Outlook.OlItemType.olMailItem);
    Outlook.UserProperty mailUserProperty = mailItem.UserProperties.Add("MyCustomProperty", Outlook.OlUserPropertyType.olText, false, Outlook.OlFormatText.olFormatTextText);
    mailUserProperty.Value = "This is from my procedure";

    mailItem.Display(true); // Pass 'true' to make the window modal

    outlookApp.ItemSend -= new Outlook.ApplicationEvents_11_ItemSendEventHandler(OutlookApp_ItemSend);

  }
  catch (System.Exception ex)
  {
    Forms.MessageBox.Show("An error occurred: " + ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Stop);
  }

}

The email that comes to my Thunderbird client has this:

X-MS-TNEF-Correlator: 000000006369751F17599447AC12B6C7A882F623A4732100


------=_NextPart_000_0001_01D9BE1B.AE7FB620
Content-Type: application/ms-tnef;
	name="winmail.dat"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
	filename="winmail.dat"

And here is the email content of the regular email (without listening to ItemSend event)

------=_NextPart_000_0005_01D9BE1B.B597D000
Content-Type: text/html;
	charset="us-ascii"
Content-Transfer-Encoding: quoted-printable

It seems that something is changing the email content after/before ItemSend event is attached to it. Can anyone confirm this?

UPDATE

I looked around a bit and found this statement:

> How to control TNEF in messages
>
> You can control TNEF by using one of three methods:
>
> Global - If you change your default email format to plain text or
> HTML, it helps make sure that TNEF is not sent unless an Outlook
> feature needs it.

How to control TNEF in messages

Can it be it: "unless an Outlook feature needs it"?

If you set a user property, Outlook will switch to TNEF since it is otherwise impossible to preserve it in a pure MIME message.

If you need to set a custom property without forcing TNEF/RTF format, set it using MailItem.PropertyAccessor.SetProperty. You can use the same DASL property name as the one generated when you used UserProperties.Add. You can see the DASL property name in OutlookSpy (I am its author) - select the sent message with the user property, click IMessage button, select the property, see DASL edit box.

Actually, the problem was because of the user property on that email:

  Outlook.UserProperty mailUserProperty = mailItem.UserProperties.Add("MyCustomProperty", Outlook.OlUserPropertyType.olText, false, Outlook.OlFormatText.olFormatTextText);
    mailUserProperty.Value = "This is from my procedure";

After I removed these lines of code everything is as before.