In message , at
02:08:14 on Wed, 19 Dec 2012, Mathew Newton remarked:recipient's mailbox, or that it has been opened/read.
That depends on how they collect their mail. If I send such an email to my own local email server, the only thing stopping it appearing in one of the recipient's mailboxes (barring the internal network failing) is if they've switched off their email client.
But even if you send someone a telex for FAX, with delivery confirmed, you can't guarantee it'll ever be taken off the machine and given to anyone.