How to resolve Office 365 message headers issues?

When you send messages in Office 365, you may encounter the following errors:

  • Headers too large.
  • Action not taken, too long header.


Occasionally, Office 365 users might have noticed that EOP (Exchange Online Protection) can't remove the X-Microsoft-Exchange-Diagnostics-untrusted header from the messages. This affects negatively on the message delivery due to oversized message headers. This is referred to as EX107354 by Microsoft.

We are recommending a temporary solution to create a new transport rule that removes the header X-Microsoft-Exchange-Diagnostics-untrusted from the emails.

Follow the steps below to fix this issue:

  1. Sign into the Exchange Admin Center
  2. Navigate to the Mail Flow tab and select Rules. Click the plus (+) icon and select Create a new rule.. option.

    Create New Exchange transport rule

  3. Provide a name for the rule, say Remove the X-Microsoft-Exchange-Diagnostics-untrusted header and click on More options..
  4. Click on the Apply this rule if... navigate to A message header.. and select includes any of these words.

    Add new condition to message header

  5. Enter the name of the header X-SigsyncProcessed and the make the text included True;

    Specify header name

  6. Now add a new action under Do the following... by navigating to Modify the message properties… and select remove a message header. Enter the name of the header as X-Microsoft-Exchange-Diagnostics-untrusted. Keep all other fields as they are and click on the Save button to save the rule.

    Remove message header

    Add message header

  7. The new rule will be placed at the bottom of the list, with the lowest priority possible. To align the new rule directly below the Sigsync rule, move it up by clicking the arrow button. By default, Sigsync Transport rule is set to priority 0 - you should create a new rule with a priority of 1. You can edit the rules and set the priority manually.

    new rule with lowest priority

  8. Wait for the changes to reflect.


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