On multiple accounts, we are receiving this message from sending to our clients which we have been emailing back and forth for a long time with no problem up until November, 2016.
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain -------- by cluster2.us.messagelabs.com. [216.82.241.243].
The error that the other server returned was:
553-Message filtered. Refer to the Troubleshooting page at
553-http://www.symanteccloud.com/troubleshooting for more
553 information. (#5.7.1)
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All clients using messagelabs (symantec) are no longer able to send to us or recieve our mail. We have had to clear up this issue with several clients by contacting their IT departments through other means and personal emails, and having each of them manually whitelist us.
If you could pinpoint the root cause of this or help in assisting us with having this block removed it would be very helpful, I can send you anything you need. It's very unusual that we've gone from being able to send back and forth for over a year to completely blocked for several clients.
Thanks,