For security reasons, FileMaker will never see the password on Active Directory. It will only send a hashed password and receive an authentication approval.
What I do is have a company directory with staff names and emails. I fill that in the field for who is sending the email. But I then authenticate FileMaker sending email with a special account I setup that only FileMaker uses. Actually, I used to use it on the same server, set it up mail service and send as auto relay and only allow it to send from the same machine (allow auto relay only to 127.0.0.1). That way I didn't even have to worry about a password. But you can do it the normal way by setting up an FileMaker Active Directory and mail account for sending out email and it could do this while showing the staff person's return email in the email field. Of couse FileMaker has officially said it won't support FileMaker Server if there are other services running, so technically you're not supposed to run a mail server on the same machine anymore and would need to use the company main mail server to send.