I only have 1 email account but it is acting as a catch-all for spam at my domain

I only have 1 email account but it is acting as a catch-all for spam at my domain

I only have 1 email account but it is acting as a catch-all for spam at my domain.   for example, my email address is john@lowcostwebsites.com.au  but I'm getting spam bouncing back at wordpress@lowcostwebsites.com.au

This second address is invalid.  

How do I stop emails to non existant accounts from being forward to my personal account?

thanks,

John.