Returned mail- User unknown

[forum-admin]

Why do I get so many email messages with the subject “Returned mail: User unknown” (or similar)?

[forum-admin]

The subject of these bounced messages is often similar to:
Returned mail: User unknown
Returned mail: see transcript for details
Returned mail: delivery problems encountered
Mail Delivery Problem
Delivery Status Notification

The body of the message may be similar to the following:
----- The following addresses had permanent fatal errors -----
< username@email!address.com >
(reason: 550 5.1.1 < username@email!address.com >... User unknown)
In the above example, username@email!address.com was an address that is no longer in use, so the system at email!address.com sent a message explaining that the email was not received.
As an analogy, this is similar to someone going to the post office, sending a letter to a non-existent location and putting your address as the senders. When the post office finds out that the letter cannot be delivered to the intended recipient, the letter will be returned to you, since your address was listed as the sender.

There are various reasons why these types of emails are coming into your Inbox but the most likely reasons are:
1. Someone has "spoofed" your email address, making it look like the email originally came from you. This is a popular tactic that spammers use to send their spam emails. If any of the spam messages are sent to an address that does not exist, the email is bounced back to your account, since the spammer made it appear as though the email originated from you. In these cases, there is not much you can do to completely eliminate the problem because the spammer has simply guessed your email address and has inserted it into the From: field, making it look like these emails are coming from you. The best thing to do if you are receiving these types of emails is to use the Report As Spam feature in Comcast Webmail or send them to missed-spam@comcast.net if you are using an email program.
2. Another possible reason for getting these emails is that a virus and/or worm may have infected your system. Make sure to update your anti-virus and firewall software and run a full system scan. If you do not have an anti-virus or firewall program installed on your computer, you can take advantage of the free McAfee VirusScan/Personal Firewall Plus/Privacy Service products offered to all Comcast customers (visit http://www.comcast.net/security/mcafee to install or for more information on these products).
If you are certain that your PC does not have a virus, the more likely scenario is that an email virus and/or worm may have infected one or more of your friends computers, and these people probably have your email address in their address book. The virus tries to spread itself by sending messages to many other users, using the address book of the infected computer. The virus/worm will often use a false address (yours) as the Reply-to: or From: address. If any of the messages that the virus sends bounce back, you will receive that message.