MS Exchange Breaking Inbound Email DKIM by Altering/Adding Headers

Microsoft Exchange is a widely used email server software that provides various features for managing email communication within an organization. However, some users have reported issues with MS Exchange breaking inbound email DKIM (DomainKeys Identified Mail) by altering or adding headers. In this article, we will discuss what DKIM is, why it is important, and how MS Exchange can affect DKIM signatures.

What is DKIM?

DKIM is an email authentication method that allows the recipient to verify the authenticity of an email message. It uses cryptographic signatures to ensure that the email has not been tampered with during transit. When an email is sent, the sending server signs the email with a private key, and the recipient server verifies the signature using a public key published in the DNS records of the sender's domain.

Why is DKIM important?

DKIM helps prevent email spoofing and ensures that the email you receive was indeed sent by the claimed sender. It adds a layer of trust to the email communication process, as the recipient can verify the integrity of the message and confirm that it has not been modified by any unauthorized entity.

MS Exchange and DKIM

While MS Exchange supports DKIM signing for outbound emails, it has been observed that it can alter or add headers to inbound emails, which may break the DKIM signature. This happens when MS Exchange processes the email and modifies the headers before delivering it to the recipient's mailbox.

One common scenario is when MS Exchange adds a "Received" header to the email, which includes information about the server that processed the email. This addition can invalidate the DKIM signature, as the email's original headers have been modified.

Another issue arises when MS Exchange alters the "From" header of the email. DKIM signs the entire email, including the headers, and any modification to the headers can result in a failed signature verification. This can happen if MS Exchange rewrites the "From" header to include the sender's display name or organization name, which can lead to DKIM failures.

Impact on Email Delivery

When MS Exchange breaks the DKIM signature of inbound emails, it can have several consequences:

• Emails may fail DKIM verification at the recipient's server, leading to potential delivery issues.
• Receiving servers may mark the emails as suspicious or spam due to the failed DKIM signature.
• Emails may be rejected or flagged as untrusted by certain email security systems.

It is important to address these issues to ensure reliable email communication and prevent any unnecessary delivery problems.

Workarounds and Solutions

If you are experiencing issues with MS Exchange breaking inbound email DKIM, here are some workarounds and solutions you can try:

1. Disable DKIM signing on MS Exchange for outbound emails: By disabling DKIM signing on MS Exchange, you can avoid any conflicts between the inbound and outbound email processing. However, this means that your outbound emails will not have DKIM signatures.
2. Implement a third-party DKIM signing solution: You can use a third-party DKIM signing solution that sits in front of MS Exchange and handles DKIM signing for both inbound and outbound emails. This can help maintain DKIM integrity without relying on MS Exchange's built-in functionality.
3. Configure MS Exchange to preserve original headers: You can configure MS Exchange to preserve the original headers of inbound emails, preventing any modifications that may break the DKIM signature. This can be done by adjusting the transport rules or using third-party tools.

It is recommended to consult with your IT department or email server administrator before implementing any changes to ensure they align with your organization's email security policies and requirements.

DKIM is an important email authentication method that enhances the security and trustworthiness of email communication. However, MS Exchange's behavior of altering or adding headers to inbound emails can break the DKIM signature, leading to potential delivery issues and mistrust. By understanding the impact and implementing appropriate workarounds or solutions, you can ensure reliable email delivery and maintain the integrity of your email communication.

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