Cheek.Org bounces back email I never sent

"I've been getting emails bounced back from Cheek.Org because they were undeliverable or the mailbox was full or something like that. The problem is that I haven't been sending any email to Cheek.Org, or at least nothing like what is in these messages. Some contain spam or even viruses!"

The Cheek.Org mail server will return mail due to several reasons, but like all mail servers it's pretty dumb and believes what the mail headers tell it. If the mail headers claim that you sent the message, then Cheek.Org will return the undeliverable message to you, whether you actually sent it or not.

It is common for spammers or virus writers to "spoof" or fake the email headers to hide where the email is actually coming from. They will often take the spoofed addresses from the same list of victims that they are using to send messages to. Victim #1 gets email that appears to be from Victim #2, while Victim #2 gets a similar email that appears to be from Victim #1.

This also happens in cases of viruses. Some viruses are spread by email and use the victims' own email address books to find new victims. They spoof the headers so that it's difficult to figure out exactly which computer is infected. Victim #1's infected computer may send email to Victim #2 which appears to be from Victim #3, who gets the blame even though he didn't do anything. Indeed, Victim #3 may not even know Victim #2; they just share a common friend in Victim #1.

Recently, spammers and viruses have started skipping the middlemen and are themselves sending fake emails which pretend to be bounce messages from other people.