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Who is Mailer Daemon and why is he sending me e-mail?

Did you get a message from Mail Delivery System<>, or System Administrator, saying something about the message failing? Possibly the Subject says something like:
  • Undelivered Mail Returned to Sender
  • Undeliverable Mail
  • Returned mail: see transcript for details
  • Failure Notice
  • Message Delivery Failed
There are two possibilities for why you would get a message like this.

1. You sent an e-mail to someone and it didn't go through. Mailer daemon is letting you know about the failure.
2. You never sent any e-mail to the address it's saying the delivery suposedly failed to. In this case it's a fake message sent by a virus.

I will cover the first possibility first.

If it is a Legitimate Failure Notice

Mailer Daemon is a process on a mail server, kind of like a mail-man robot delivering your e-mail. Or trying to deliver your e-mail, because when you get a legitimate one of these "Delivery Failure" messages, it means that he tried to deliver it and couldn't for some reason. So, these messages are kind of the e-mail equivalent to getting a letter back stamped "Return to Sender". If you read through the e-mail from Mailer-Daemon, it should try to explain why the message couldn't be delivered, mixed in with a lot of technical jargon.

Important: A real mailer-daemon message will not have an attachment, unless you sent out a message with an attachment. Attachments are dangerous! If you get a failure-type message with an attachment on it be very careful. It very likely could be a virus.

Now I'll show you how to decipher what the mailer daemon is trying to say. Here is a sample message:

The original message was received at Tue, 21 Feb 2006 17:49:39 -0500 (EST)
from []


Your e-mail is being returned to you because there was a problem with its
delivery. The address which was undeliverable is listed in the section
labeled: "----- The following addresses had permanent fatal errors -----".

The reason your mail is being returned to you is listed in the section
labeled: "----- Transcript of Session Follows -----".

The line beginning with "<<<" describes the specific reason your e-mail could
not be delivered. The next line contains a second error message which is a
general translation for other e-mail servers.

Please direct further questions regarding this message to your e-mail

--AOL Postmaster

----- The following addresses had permanent fatal errors -----

----- Transcript of session follows -----
451 <>... reply: read error from
451 <>... reply: read error from
451 <>... reply: read error from
... while talking to

>>>>>> RCPT To:<>

550 <>... User unknown

Reporting-MTA: dns;
Arrival-Date: Tue, 21 Feb 2006 17:49:39 -0500 (EST)

Final-Recipient: RFC822;
Action: failed
Status: 5.1.1
Remote-MTA: DNS;
Diagnostic-Code: SMTP; 550 MAILBOX NOT FOUND
Last-Attempt-Date: Tue, 21 Feb 2006 17:49:58 -0500 (EST)

The important line in this message is where it says:

<<< 550 MAILBOX NOT FOUND 550 <>... User unknown

In other words, for some reason AOL couldn't find this account "wfe76trg4" in their system, or at least couldn't deliver to it. Possibly there is a typo in the address, possibly this person cancelled their account with AOL and switched to some other provider, or possibly they are just late paying their AOL bill this month. So check into these possibilities; maybe call the person by phone and ask if they changed their e-mail address.

The above is just one of many different things that this explanatory line could say. If it says "Mailbox Full", for example, that means the person you're sending to has run out of room in his email inbox. Call him and tell him to delete some of his old junk mail to make way for your important messages.

If it is a Fake Notice From A V irus

There are many viruses around on the internet that send out fake failure notices out to everyone they can, trying to trick you into opening the attachment and infecting your computer. If you get a failure notice for an email you don't remember sending, possibly going to some email address you've never heard of, you've probably just received one of these fake messages. Also, a real mailer-daemon message will not have an attachment, unless you sent out a message with an attachment. Do not open the message, and definitely do not open the attachment. Just delete the message. Everyone gets these occasional fake messages, so don't worry about it. Visionary's virus detector should filter them out for the most part, before they come into your e-mail program.

If you are getting a lot of these messages -- as in hundreds every day -- then the problem is closer to home. There are two possibilities:

1. Your virus software is up to date and your computer is not infected. However, someone's computer listing you in his or her address book is infected. Some viruses/worms use the infected computer's address book entries to send itself to all the person's friends and thus increase the chances someone will open the attachments. Simply delete all of the delivery failure messages and let your friend know it looks like they are infected.
2. Your computer is infected by a new mass mailing worm and is sending out numerous emails. All of the receiving parties' virus scanning systems are rejecting these messages and returning them to you.

So, if you are running anti-virus software make sure your definitions are up to date and run a virus scan! If you are not running anti-virus software, and you are getting hundreds of these failure notices each day... that is a very bad sign. You are probably infected with a virus and will need to clean it out. Click this link to visit a web page and download a free removal tool:

You could also run a free scan at .

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