Are you getting tons of undeliverable email notices because of emails you didn't send?
One of two things may be happening:
1. Your email address is being used by someone else.
If your address has been hacked, change your password immediately. Your password needs be changed in cpanel as well as in your email program (Outlook, for example).
2. Your email address was put in the 'from' address (spoofed) of someone sending spam.
If your address has been spoofed, changing your password won't help as the email isn't coming from your server. Rather, add an SPF record to restrict email using your email address to be valid only if it comes from your mail server.
You can create an SPF record for each domain on which you use email. Just keep in mind that if you use any outside sources to send email in your name (like a hosted shopping cart, for example), you need to add that IP or domain into your SPF record too, as a valid sender.