To set custom adhoc email options for specific group, go to the Settings page of the group you would like to work with.
in CUSTOM EMAIL OPTIONS it is possible to set custom To, From, Reply to, Subject Prefix, Footer.
Please note, by default an unsubscribe link, a topic link as well as the group's email address are automatically populated in this box.
You, of course, have the option to delete any and all information in this box.
Please see the following article about how to set custom adhoc options at the network level.