How do you install default mail client?
A default email client is an email program such as Thunderbird. When you click on a link anywhere to send an email it goes to that program. Windows has a setting for default programs. In order for a link on a website to use the Yahoo mail website rather than a program it must override the normal action. I know that Firefox has an option to pick an email website rather than the email program. That does not alter non browser actions such as trying to send email from a scanner.