The problem: You want to send a newsletter to each of your customers. You go into Outlook, compose an email and add the recipients addresses to the "To" box. Then you press send. Each recipient receives an email, BUT the addresses of the other recipients are visible.
You could use the BCC field and send the mail to yourself, but this isn't great either. Ideally you'd compose one email for each recipient so they all get delivered separately and individually. This could be a long process.
The solution: One by One. Just compose an email and hit "Send One by One". The email will be sent to each of the addresses in the "To" box individually. They'll never know you sent the same email to the other people.