Is an e-mail program is used for one email delivery only?
What happens if I need to resend it? Do I need to open a new email program?
Are you using Email Send or Email Default?
Email Send should be used once. If you need to re-send, clone it and make sure you edit the smart list.
Email Default - flows can be re-used if you set the Qualification Rules to Every Time or another value. BUT, i highly recommend to clone the flow and update the smart list so you can back track issues better.