add html checkbox via outlook addin (c#)

I create a mail in outlook via an addin (VSTO).(request for instrument maintenance) I want to use checkboxes for the customer to check if he wants exta service.`

Microsoft.Office.Interop.Outlook.Inspector inspi = this.Context as 
Microsoft.Office.Interop.Outlook.Inspector;
Microsoft.Office.Interop.Outlook.MailItem openMail = inspi.CurrentItem as 
Microsoft.Office.Interop.Outlook.MailItem;
[...]
openMail.HTMLBody += @"<input type = ""checkbox"" name = ""item1"">Extra1<br>";
openMail.HTMLBody += @"<input type = ""checkbox"" name = ""item2"">Extra2<br>";
openMail.HTMLBody += @"<input type = ""checkbox"" name = ""item2"">Extra3<br>";

But no real checkboxes are created, only squarebrackets with space in between.

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Can I create real checkboxes via VSTO?