@HL I did this for a client a while back with a combination for workflow and email capture scripts.

Was the script necessary to send the approve/rejection action via email?

The script was required to receive the reply from the user yes. Its a pretty error prone process because the email can contain any text values and may or may not contain the string(s) you want. For instance if you look for "approved" and they write approve you will not get the approval or if they write NOT approved meaning to decline it you will still pickup the word approved. So you just have to be careful with how you do it and they have to be disciplined in how they respond.