There is an option in the Manage Email Defaults screen to "Use document creator's Email Address"
This option uses the "Business Email Address" field (if available) for the Staff member as the Default Sending Email address.
This means that emails sent through the email queue by each staff member, would use the Business Email Address for that staff member, if one is present. Otherwise, it would use the sending addresses set up in the Email Defaults.
Note, that not every email server will be compatible with this function. For example, if your email server requires a specific login and you have to enter this in the Username and Password fields of the Email Defaults, you would need to make sure that the login you’re using has the ability to send from each of the email addresses your staff would be using in the Business Email Address field.