This issue occurs because Microsoft Exchange changes the headers of a message, and this breaks the DomainKeys Identified Mail (DKIM) signature.
To resolve this issue, install the following cumulative update:
2961810 Cumulative Update 6 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the DKIM signature that is defined in RFC 6376, go to the following website: