Organizations unable to email us

Good Morning All,

We have an organization that is unable to email us even though we have white listed them. I can email them but they can not email us. Error message on the bounce back email they get is below. Please help.

<myemailaddress>: host cluster6.us.messagelabs.com[67.219.250.196]

    said: 553-Message filtered. Refer to the Troubleshooting page at

    553-http://www.symanteccloud.com/troubleshooting for more 553 information.

    (#5.7.1) (in reply to end of DATA command)