I want to send HTML email from my Outlook or Yahoo, Gmail, Hotmail. I wasn't able to find any such option where I could write HTML which will then be rendered at the receiver's end.
How can I do that?
If you want to send HTML emails, you need to use a tool that let you write the content of the email yourself instead of relying on WYSIWYG editors. The most part of mail clients doesn't allow this feature, mainly for security reasons.
With web email clients, such as Gmail, Yahoo or Hotmail, you can try to use the WebKit Inspector (in Safari and Chrome) or Firebug to edit the content of the email directly in the DOM, bypassing the interface restrictions. However, your code is likely to be sanitized by the application and it won't work as expected.
The best solution is to send the emails using a programming language (e.g. mail() in PHP or Pony in Ruby), or a mail service designed to handle HTML content such as MadMimi, Mailchimp, Campaign Monitor...
In Thunderbird, there is a plugin called stationery which lets you see the HTML of an email and edit it. Perhaps that's what you're looking for?
You can make your HTML content ready in your favorite editor and test it.
Then you can use "Insert > HTML" in Thunderbird to insert that HTML content in to the mail.
Embed html document in Microsoft Outlook
Save the file as an HTML file, When you select the HTML file the INSERT button right will activate - but to the right of the button there will be a down arrow,click on that,and you will see two options - INSERT and INSERT AS TEXT - CHOOSE INSERT AS TEXT - and your HTML will appear within the Email box!
HTML in email clients is a very very thorny problem because most of them do not just embed a browser, but implement random subsets of the HTML and CSS. And there is a LOT of email clients.
If you really do need to do this, you may look at professional solutions from Mailing List Marketing companies. It is a known problem in that space and there are services, tutorials (example) and checklists for that. Here is a service from MailChimp. Most of the providers offer something similar.