If you are receiving bounce-back emails, but did not send emails, it means that a spammer has been using your email address to send spam emails.
There are three possible ways this can happen:
Your email address is being spoofed
Spammers can make it seem as though their emails are coming from your account by putting your email into the “From” field.
This unfortunately cannot be stopped. If it does get to be too out of hand, you could change your email address.
Your mailbox has been accessed
Your password may have been determined and spammers might have accessed your mailbox to send emails.
You should change your password immediately, using password generation standards to create a password that is strong and unique.
Your computer has been accessed to get into your mailbox
You may have a malware program on your computer logging passwords and other information.
You should run antivirus or malware scans on all computers used to access your mailbox, and you should change your passwords on all your systems to strong and unique passwords.