Create an SPF record to show that your domain does NOT send email

Nothing stops spammers from using your parked domains to send email.

I had a client who was seeing this and there is simple way to let spam filters know that your parked domain doesn't send email.

Simply add this as a TXT record

v=spf1 -all

This is an SPF record that says to always fail.

The SPF Project has more details on the SPF syntax.