In the send modal, Select which customers is a multi-select of the customers attached to the proposal; only the ones you highlight receive the email. If a customer's account has no email address on file, the send for that customer silently fails (nothing is queued for them) while everyone else you selected still gets their email. If a customer is missing from the list entirely, add them under Select Users on the proposal and Save before sending. Attached files must be pdf/jpg/jpeg/png, otherwise the whole send is stopped before anything is emailed, with the error "Invalid file type. Please upload valid files."