https://portal.treepl.co/backlog/3022
This is a feature our clients have complained about numerous times since migrating to Treepl CMS, when they receive a workflow notification, they are not able to respond directly to the submitter, they have to copy and paste their email address manually.
Request to automatically populate workflow replies with the submission email address.
@Peter-Schmidt could you please update backlog request 3022 with this forum post
Sure - added 
https://treepl.co/public-backlog-state/request/automatically-populate-reply-field-in-workflows
For the time being maybe you can use this workaround, worked for me 
@Peter-Schmidt This looks like it has already been implemented and can be removed from the backlog. Current workflow email "reply-to: " is the submitter’s email address.
Thanks @Rhatch - The team must have missed this. I have moved it to “Delivered”