If you suddenly find that you are receiving many Mail Delivery Failed emails from addresses to which you know you have not sent emails, most likely your email account password has been stolen and spam has been sent from it.
What to do if your email account password has been stolen
In this case, urgently change your email account password (from cPanel/Email Accounts) but do not log into your email client or webmail because the trojan on your computer will send the new password.
You need to perform a full scan of your computer using an effective antivirus (free antiviruses will not protect you from such problems, not even many licensed antiviruses). Our advice is to download the trial version of Bitdefender or MalwareBytes and scan your computer.
After a full scan and removal of trojans and viruses, you can log in to your email again.