Why do I get bounced mail which i never sent ?
Many spammers and viruses will automatically select random email addresses to send emails to (to promote their product or to infect people's computers). They will also randomly select addresses in the same way to claim the email is being sent from that address (known as Email Spoofing). This is done to make the claimed sender address appear (or actually be) more legitimate and look less suspicious.
Unfortunately, the spammers use open relays (servers that don't require authentication) to send emails through, and viruses often use their own built-in SMTP servers to send the viruses out from. Thus, as the fraudulent emails originate from another source, we have no way to prevent this from happening, nor do you.
If you suspect that you are infected with a virus, you should obtain the most up-to-date anti-virus software to run a check on your system just to be safe (especially, if you use Outlook on Windows, as that is the most popular email program for email virus writers). Also, you should always use well chosen passwords for your account and email that are not based on dictionary words or are easy to guess or otherwise are simple, which will prevent your email accounts from ever being compromised just for such reasons.
NOTE: The best method to prevent bounce back emails from spammers and viruses, is to disable catch-all email in your control panel.