Yahoomail is my default mail service internet option but do not work fully?

hi. when i open an eamil address in a site internet , it does open properly the yahoo mail service "compose" with my email address in the "FROM" section, but the "TO" section is empty instead of having the address. how can I resolve this problem? thank youYahoomail is my default mail service internet option but do not work fully?
If the 'insert address' located above "TO" is clicked the address book opens when you can check the address required and give the "insert the checked address" command. The checked address is inserted immediately and the click the 'send' button when the mail is sent immediately.