I assume you are using the Send Mail script step. In that step, you need to uncheck/unselect the option to "perform without dialog". With it set like this, it will open up a new message in the Mac mail application. With the message open, your users will need to choose which mailbox they want to send from. See attached screenshot. I don't have the drop-down open, but I too have several mailboxes setup and they all appear in this menu.
Choose from address.png 78.9 K
Thanks. I did and it works..