Darwin, I recently configured and implemented the email listener for exactly the same use case.
When the PO gets submitted for approval it sends an email to the approver with a copy of the PO Details report attached telling them they will be sent an email to approve or cancel this PO. This is then followed by an email which has two options for them.... respond with a 1 in the body of the email to approve the PO, respond with a 2 in the body of the PO to cancel the PO. When their email is received and processed by Maximo they will then receive an email telling them that the PO has either been approved or cancelled depending on what their choice was.
The reason for the two emails to begin with was a specific requirement to send an email with the PO attached.
I am going to be providing step by step instructions on how to do this at the next MaxTECH User Group meeting on March 31st, if you are not able to attend that meeting in person I will be happy to send you the instructions I will be providing. This was done in a Maximo 7.6.1.3 environment using MS Outlook and OAuth 2.0 authentication for the listener.
regards,
Steve Hume