Hi Michael,
Bob is right, Mail is not going to interpret your documents layout in the same way you have created them in Pages.
One way is to recreate the newsletter in Mail as a template that can be edited easily to closely match the newsletter format.
Another is to ensure the newsletter is only one page long and it will then preview in Mail as an image in the email body but will require opening in an email reader for URLs to work.
You do have to understand not everyone you are emailing has the same setup as you do, they may only look at email on their iPad, Phone or PC perhaps.
There is no way without extra software or an online builder for you to get the same layout of your newsletter to work in an email. A newsletter is a specific width and height of page where an email has the ability to flow and adjust to conform to the size of the window it is being viewed in. The text must be fluid and not as restricted as a printed page which has dimensions to conform to so it fits onto a certain paper size. Your email can change based upon how wide your screen is but when printed it will also conform to the size of the paper it is instructed to fit on by your Page Setup.
I would love to recommend a free builder but I am not familiar with any of them, sorry
Best of luck