How and where to place - css, html and images in email to make it a HTML email?
I have seen many advertising emails in my inbox and I am curious to know how does the email body display the HTML.
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Take a look at
If you are even more curious (and have minimal developer skills) take a look at libraries like PHPMailer to see how HTML mails can be built.
Images you must hot link to, CSS will be inline styled. Few styles are rendered by email clients such as gmail yahoo aol outlook etc. Make sure to use tables and not pure CSS columns etc.
I would look at MailChimps knowledge base which have great articles on this http://kb.mailchimp.com/article/how-to-code-html-emails