Postmaster Delivery Status Notification
If you're sending a lot of emails - especially if you are typing in the address manually - you will sooner or later get a postmaster notification. The delivery status notification tells you who could not receive the mail sent and returns a copy of the mail.
I'm not an expert at mail servers (never tried setting one up in order to test how it works), and I've only received a notification from Hotmail. I guess the postmaster service at Hotmail will notify you if and when there is a problem. Point is, this error message might not be displayed in all systems.
Anyhow, I started receiving a ton of delivery status notifications on my secondary Hotmail account (disposable email I've been using for years, 20 spam per day) and decided to check it out. The postmaster notified me several times about a few email addresses and the email attached was similar to other spam messages I've received. Conclusion: I was hacked.
The emails' originating IP was not close to the one I was using, so it had to come from somewhere else. My PC was not compromised, but my secondary Hotmail account was. I swiftly changed my password, and I stopped getting those postmaster delivery status notifications.
This is a perfect example on why not to use the same password for your email and shady services. Obviously some forum went rouge and sold their users passwords and emails.