To set-up different 'reply to' and 'send' email addresses, all you need to do:
1. Create a user and log in as that user via Passwords
2. Note the user name, let's say it's KARL JONES
3. From front menu select Toolbox|Staff Profiles
4. Add a new staff Member, Karl Jones.
5. Under "Email" set the email address for that user.
6. Save the record.
If you are in company 'a' the Email address for Karl Jones in company 'a' will be used when sending email, based on the Staff Profile. If you are in company 'b' the Email address for Karl Jones in company 'b' will be used when sending email.