If the recipient taken from the SMTP envelope ("RCTP TO") is not included in the CC list ("CC" email header), some mail servers add this header as an aid to clients which would otherwise not be able to display the envelope recipients.
"Envelope-To" is not a standard email header.
You can access any email header using IMail.Document.Root.Headers collection:
IMail email = new MailBuilder().CreateFromEml(eml);
string header = email.Document.Root.Headers["Envelope-To"];
We plan to add "Envelope-To" and "X-Envelope-To" support as regular properties in the next release (IMail.EnvelopeTo, IMail.XEnvelopeTo).