Problem Explanation Problem I get a 'mail undeliverable' error for mails that I have not sent and I don't have viruses on my computer. Why is that? Explanation Some viruses (such as Klez) can search the address book and cached web pages of the infected machine and propogate itself to the address that are found on that machine. They also forge the sender of these messages by changing what appears on the "From:" line of the email. Thus, even though your machine is not infected, if your email address is in the address book/cache of an infected machine it is possible that an email has been sent with your address as the sender. If there is some problem with the "To:" address of this message, the automated delivery agent would "bounce" the message back to you since your's is the only address available to it. What to do about it? Please forward the entire message with the complete headers to How to get complete headers of email messages? Eudora: In the incoming message window, click the 'BLAH BLAH BLAH' button at the top left corner of the window. This will display the complete headers and then forward the message to . MS Outlook: Select the message in the mailbox. Click on the 'File' menu and select 'Properties'. In the 'Properties' window, click on 'Details' tab. The complete message header for the corresponding message can be viewed. Copy and paste this header before forwarding the message to .