To prepopulate email body instead of manually typing the reply, we can make use of "mailto:" function to generate a button. Using this will result in a new email instead of direct reply.
You can modify the code content to generate your desired reply. Then modify the listener content pattern in Email Polling System Demo App to be able to read your reply.
<a href="mailto:#appVariable.From#?subject=Re:%20#envVariable.EmailSubject#:%20Desc:%20#form.j_ave_transaction.Description#&body=Approved%0A%0A ID:%20#assignment.processId#">Approved</a>
Details:
#appVariable.From# is the sender email.
#envVariable.EmailSubject#:%20Desc:%20#form.j_ave_transaction.Description# is the entire email subject.
Approved%0A%0A ID:%20#assignment.processId# is the body content of the email reply.
Please note that you must use "%20" instead of spaces and "%0A " instead of Enter in the subject line and body. To create the text with "%20" and "%0A" you can use this page https://www.urlencoder.org/