If it is 'undeliverable mail - return to sender' it would seem to indicate that you are the sender it is being returned to. Time to do all the scanning for trojans etc that is so often recommended in this forum.
I have similar in my spam folder with gmail, all msn related apparently, gmail tells me
Sometimes you may receive a message intended for someone whose address resembles yours but has a different number or placement of dots. For example, your address might be [email protected], but the message was sent to a [email protected]. What's going on?