Outlook can only do that when it is attached to an exchange server. It is an enterprise thing. Note that verisign was a company that issued signing certificates. Symantec ran the company so well their certificates were distrusted by all the major browsers and it's certificate business was ultimately sold. So I am a little confused as to exactly what you are talking about with your use of verisign Internet mail, of which Thunderbird is a client allows you to request delivery receipt and read receipts, but unlike the corporate systems returning read receipts is entirely up to the recipient. You can not force the issue. There is still no real substitute for paper where enforceable legal documents are concerned. Any digital record where one of the parties has access to edit is subject to question and expensive forensic analysis if it is challenged.