You have likely experienced the irritation of receiving a message from the mailer daemon in your email inbox. Emails are sent to users from this address when emails previously sent are undeliverable. This is often because the recipient address was incorrect. Be wary, however; some emails that look like they are from the mailer daemon are actually from users spreading viruses. Never download an attachment from one of these emails. If you wish to permanently block mailer daemon emails, you have several choices. Check your blocked emails folder regularly to ensure all of your important emails have, in fact, been sent.
Double-check the email address of the recipient against before you send an email. Many times, a simple typo will incite an endless stream of mailer daemon returns. Make sure you have typed the address correctly before clicking send.
Download an email address verification program such as G-Lock or Postfix. Each time you send an email, an address verification program will confirm that the recipient address is good before the email actually goes out. If the address is bad, the program will prevent the email from sending and notify you; as a result, no mailer daemon notifications will be received.
Block the mailer daemon email address. This is an especially good idea if you believe the notices are fraudulent. Follow the directions for your particular email provider to add its mailer daemon address to your list of blocked addresses.