If you wish to make adding a Reply To address mandatory at the point of sending an email, you can ask your AM to set this at Admin.System.Emails.Require Reply To.

You can also specify what you would like the default Reply To address to be, to prevent the Reply To being forgotten, and to save your users from entering the address each time.

If you set a default Reply To address you can still overtype with a different Reply To address at the point of Send, if you choose.

Please note: The pre set Reply To email address will appear for any newly created email campaigns.