Gmail/Yahoo Not showing Bold text

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

Your email appears to be "text" not "html"
Text emails ignore the formatting

So where to do the change , individual change in customer section or admin -> configuration -> email options -> enable html emails to true , currently set false .

Thank you very much for quick reply

diptimoy:

So where to do the change , individual change in customer section or admin -> configuration -> email options -> enable html emails to true , currently set false .
Yes that's correct

and if you are testing by sending to an admin email address change
**Email Admin Format?

**Now its up to to receivers email client as to whether they use text or html to display the emails

Thanks . Wil try and let you know

Thanks you very much . Changing to HTML worked .