When you send an email, you are using a MUA (mail user agent) (like Outlook) which delivers the email to a mail delivery agent (MDA) (like Exchange or Sendmail). which does the actual work of figuring out how rto deliver the email, and making the connection to the MDA at the other end.
If, when the MDA at your ISP is not able to deliver the email on the firsttry, it puts it into a queue and trys sending the email at various intervals until it gets delivered, or it cannot be delivered at all.
There is a protocol involved where the sending system says "Hello", the receiver says "I'm here' send the message", and message is sent, and the receiver says "I got it!". Sometimes, due to some network or system problem, the message gets sent, but the sender never receives the "I got it!". So the recipient gets the message, but the sender doesn't know it, and continues to retry sending the message.
It looks like your ISP didn't start trying to deliver the message for 33 hours (maybe your the user's desktop didn't connect to the ISP for that period). On the first try, the scenerio I mentioned may have happened and the message really got delivered but got requeued. At the 2nd try, the 1 day timeout was seen when the message was processed out of the queue and the NDR (non delivery message) was returned.
It was my experience when I was a corporate postmaster, that many Exchange systems had mis-configurations which could cause timeouts before the "i got it" was sent although the message was really delivered to the recipient.
