I have an error "the sender's email address is already in use" when I set the email address for sending invoices.

This error is reported because you have entered an email address already assigned to a SY user.

The Email address for sending invoices is to be filled in if the invoices must be sent by a server, or from an address different from those of the users.

You must enter a unique email address, different from the email address of SY users.