Summary: 

To set a 'reply-to' email address for your transactions:

Steps:

1. Click on the Settings module
2. Click on Company 
3. Click on the Email Templates tab
4. Select the Transaction you'd like to update
5. Edit the From Email Address
6. Click Save

Result:

When emailing a transaction PDF, the From Email Address will be seen on this email. The recipient will be able to reply to this email address. 

NOTE: The email will be include the address 'noreply@unleashedsoftware.com' to show it has been sent on behalf of, which is done for security reasons.