When you sign up for hosting, you automatically get a 'postmaster' E-Mail account. If you had set your forwarding address for the 'postmaster' account, then any E-Mail to any non-existant E-Mail account on your domain would automatically go to your postmaster account and be forwarded to the E-Mail address you specified. This is why you may be getting E-Mails to another account that was meant for your domain.

You may change your forwarding address on your 'postmaster' account if you wish or even remove the forwarding address and leave that space blank if you no longer wish to receive such E-Mails at the account you specified. Please see How To Create, Delete, Change, or List E-Mail Accounts for more information on changing an E-Mail account's information. Please also see How To Set Up E-Mail Forwarding.