Dear Pakkakul,
ISS04372165 - Email Based Approval functionality is not working ITSM 8.1
I have found KA417751 below, please refer to it for your issue.
https://kb.bmc.com/infocenter/index?page=content&id=KA417751&actp=search&viewlocale=en_US&searchid=1414041179762
Please let me know if any question with it. thanks for patience and assistance.
Regards,
John
Technical Support Analyst