Yes, you can send your custom made HTML Email using your Web-based Email account account like Gmail, Yahoo, Hotmail, etc. Many people think HTML can only be send using Email Clients like Outlook or Thunderbird, etc. To send an HTML using your Web-based Email account, you need to do make a valid HTML Email Template first, like with in-line CSS, using Tables, absolute paths etc. Read here how to write an HTML Email. After you are done making it, upload it to your server or if you dont have one you can use a free web hosting service here or any other which you like. Make sure your HTML Email has absolute paths for everything, your readers are going to be at all different locations.
When done, open your Web-based Email account, click on Compose/New Mail, make sure you have Rich Formatting enabled. Also, open your HTML Email using your Web Browser. Select all on the HTML Email page using Ctrl+A(On Windows) Or Command-A(On Mac) and then paste it on your Rich Text Editor(where you are writing the Email) using Ctrl+V(On Windows) or Command-V(On Mac). Send it. Thats its. So easy to do.
Make sure HTML Email is properly formatted, be careful with it. CSS rules vary for Web-based emails, also it is not necessary that the receivers Email Client has HTML support it, it will go as plain text in that case, and if they are not web code friendly, they might just delete it in one go thinking it as some spam. Thus, you should give the receiver the opportunity to choose between HTML Email or plain text. Also if it is a commercial Email, make sure it follows CAN-SPAM Act of 2003.