How to make e-newsletter

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I'm a freelancer working at home,recently I'm working on preparing marketing materials for new year's promotion campagin,but I don't know how to make e-newsletter.I often receive this kind of newsletters in my mailbox,they are in HTML format and when I open a new email,the content of the newsletter will be read directly and no need to open the attachment.I just wanna I can make this and send it out by email directly.Anyone can give me some tips? Many thanks!