Email NDR with error "Headers too large" or message filtered by Symantec Cloud


You are sending mail through Exclaimer Cloud back to Office 365 using connectors, you may experience issues such as –

  • 550 5.0.350 Remote server returned an error -> 553 Message filtered. Refer to the Troubleshooting page at; for more;information. (#5.7.1)
  • Headers too large (32768 max)


There is a known issue within the Microsoft Office 365 infrastructure which means that a subset of Exchange Online servers are not correctly stripping the “X-MS-Exchange-Diagnostic-Untrusted” headers from the message.

This results in bloated headers which upstream providers are not able to process and thus the email gets rejected.


There is a bug raised within the Microsoft engineering team to resolve this issue. The Microsoft SHD number for this is EX107354.

Update as of 28/6/17: The coding is in place within Microsoft to resolve this and is being tested now. We are told the fix for this will be deployed as part of a new build however the deployment process can take some time in an infrastructure as large and complex as Office 365. 


There is a work around for this issue in the short term until Microsoft deploy their permanent fix.

Please create a new transport rule in your Office 365 tenancy as below. Note, some of these settings are only available after you click “More options…” in the new rule window–



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