I replied to a “do not reply” email. Does my reply sent to its recipient?

I replied to a "do not reply " email by mistake and didn't want to send the message.  IT didn't show in my "sent" folder as being sent and it didn't show in my inbox as a fail. It dropped into "junk"  Does this mean that the email never got in to any of their inboxes and just bounced back into my junk folder or did it arrive and they will be able to see it?

Original Title: I replied to a "do not reply" email. It returned to "junk" and didn't show in my folder as "sent" Does this mean that the email never arrived in their inbox or is it somewhere waiting to be read.?