|Mailer Daemon, to my understanding, means that:|
You either sent an e-mail and the person your sendt it to is not there anymore, their mailbox is full, or an incorrect e-mail address is used.
If you are only recieving the Mailer Daemon without sending mail, you may have a trojan emailed to you, or a worm copying itself using email.
If thae latter is the case, you need to get rid of it by using a few virus scans.
Is your AntiVirus program alerting you of any malware?
Retired - Doin' Dis, Dat, and slapping malware.