User thinks he or she approved a requested item but receives an email message stating "An approval reply has failed to be processed by our system"
Approvals by reply email.
If the user does not type the word "approve" in the subject line of the reply email or does not click the Approve button in the email body, which adds "approve" to the subject line, the approval reply will fail. Other replies such as stating "I approve this" in the email body is not sufficient to approve an requested item.
The out-of-box inbound email action that controls this behavior is named Update Approval Request.