Gmail/Yahoo Not showing Bold text

[diptimoy]

I want to make some of the heading like payment , Shipping Address , delivery address BOLD , but while I view in gmail or yahoo its not showing the BOLD CHARACTER .Kindly advice . thanks in advance.

here is my language files ( location is correct ,I mean the code is running from the same file, as I change the variable values rather than they are present within English language folder , the new values are comg in mail but strong has no effect )



define('EMAIL_TEXT_ORDER_NUMBER', '<strong>Order ID:</strong>');
define('EMAIL_TEXT_INVOICE_URL', 'Detailed Invoice:');
define('EMAIL_TEXT_INVOICE_URL_CLICK', 'Click here for a Detailed Invoice');
define('EMAIL_TEXT_DATE_ORDERED', 'Date Ordered:');
define('EMAIL_TEXT_PRODUCTS', '<strong>Products</strong>');
define('EMAIL_TEXT_DELIVERY_ADDRESS', '<strong>Delivery Address</strong>');
define('EMAIL_TEXT_BILLING_ADDRESS', '<strong>Billing Address</strong>');
define('EMAIL_TEXT_PAYMENT_METHOD', '<strong>Payment Method</strong>');

see some are assigned with strong to make them BOLD but mails not showing ( the mail screenshot is attached) see I chanegd Order number to order Id its showing but not the bold text