Sometimes you may receive an email bounceback with the message, "Mail delivery failed: returning to sender," or "User doesn't exist: returning to sender" for messages you never sent.
The obvious concern is that your email account is compromised; however, this is often not the case. What happens is that spammers use your email address as the From in spam or phishing messages sent to others. That way, spammer's true identities are hidden, and they won't receive bouncebacks of potentially tens of thousands of emails. In this case, spammers did not access your email account to send the spam. They simply forged the sending address. It's as if someone wrote your name and address as the return address on an envelope and dropped it in a postal mailbox. If it can't be delivered, the envelope will be returned to you by the post office. It is very unlikely you were personally targeted.
Spammers and phishers gather email addresses in many ways. Most email addresses they find, including your University email address, are publicly available on the internet. Such emails are usually gathered automatically through scripts and compiled for later use.
Unfortunately, if it is a case of simply forging the sending address, there is nothing you can do. If the volume of these bouncebacks is high, you may wish to create a filter in your email client to automatically file these.
Also, for peace of mind, check the Sent folder of your email to see if such messages were truly originating from your account. Be aware that a spammer who did breach your account could send messages and then delete them from your sent folder.
Finally, IT Services recommends that you change your CNetID password through MyAccount (myaccount.uchicago.edu). If an attacker does know your password, the individual should not be able to get back into your account if you change it.
Email Information Security at security@uchicago.edu or call 773.702.2378 if you have additional questions or concerns.