I have the issue with new users not receiving their password emails. I asked for a working link to a possible solution in another thread, but that has not been posted yet. In trying to fix that issue, I found another (related?) problem.
When a user registers on my site, I get the notification email. 2 days later I also get one of those "MAILER-DAEMON" emails notifying me of a failure. This was also happening when someone used the Contact Form on my site. I seemed to track it down to having an auto responder on the email that the contact form and new user registration was going to. I put in a new email account without an auto responder for the contact form to use and that seems to have stopped those failure notices. I still get them when a new user registers, however.
I changed the email address on my Dashboard>Options>General field ("used only for admin purposes"),to the new email without the auto responder but I got another failure notice this morning.
Does the new user registration use that Options>>admin email or does it pull it from somewhere else?