Verified answer

Above the To: section for all outgoing mails (New, Reply, Forward) is a command to 'Use BCC'. This means Blind Carbon Copy - none of the recipients will see any address except their own. Yahoo does not show 'undisclosed recipients' although other mail services do - all it shows is one address instead.

Put their email address in the bc category, that means blind copy.

When they receive the email they will only see their email address.