Cannot send emails to messagelabs customers

One of our clients cannot send emails to messagelabs customers.

"553-Message filtered. Refer to the Troubleshooting page at  553-http://www.symanteccloud.com/troubleshooting"

I've used the lookup tool and cannot see anything wrong. I've seen a similar thread in which admins found that the IP was being throttled and were able to refresh the status.

Could you check IPs 91.227.208.0/24

and tell me if this is a similar situation?

Thanks for your time