Short answer:
Internal emails may not receive Xink signatures because Exchange Online uses TNEF format. Xink can convert these messages to standard MIME format so signatures can be applied.
When using server-side signatures, you may notice that signatures are not applied to certain internal emails.
This behavior is related to how Exchange Online formats messages sent within the same Microsoft 365 tenant.
Why signatures may not be applied
In some scenarios, Exchange Online sends internal emails using TNEF (Transport Neutral Encapsulation Format).
This typically occurs when Exchange assumes the message will be handled by another Exchange system, such as internal communication within the same tenant.
Because TNEF is a proprietary Microsoft format, the Xink ReRouter ignores these messages by default.
As a result, signatures are not applied unless the message is converted to a standard format.
How Xink handles TNEF messages
Xink includes a TNEF message handler to ensure signatures can still be applied.
When enabled, the handler performs the following steps:
- Unpacks the TNEF-encoded message
- Converts the message into standard MIME format
- Applies the email signature using the normal processing workflow
- Returns the processed message to Microsoft 365
This process is automatic and not visible to end users. Emails appear unchanged, but signatures are correctly applied.
Important information
Server-side settings are cached across the Xink processing infrastructure.
After making changes, allow some time for updates to propagate before testing.
Recommendation
If signatures are missing on internal emails, ensure that the TNEF handler is enabled and test again after allowing time for configuration changes to take effect.
