How to insert a html file in the body of an email (Newsletter)

Hi everyone,

We created a newsletter (html file) that we want to send to our customer with zoho CRM.
I want to insert the html file directly in the body of the email and not attached to the mail.
In order that customer can read it without opening the file. I tried everything but it does not work.

How can I do that?

Thank you for your answer, it will help me a lot,

Best regards