I had the same questions. However, here is how I got it done. First, the Return Actions option is correct for accessing the confirmation letter. Then it is necessary to save to local folder, i.e "2023 efile cofirmations". The you have to go to clients and select the client email. (At tat point it took quite a bit of setup to get your email onto outlook) Once the new emial pops up, your email should be populated along with the email destination for the client. Then in the subject line I put "2023 e-file confirmation notice. The choose the "Insert" option at the top, the drop down option should be "search computer" and find the local drive where you saved the letter and select "open". At that point all you have to do is choose your signature option and send.

Hope that helps.