InDesign is not even close to the right tool for this. Best you can do now is create a PDF and link to that.
Look into Constant Contact or Mail Chimp.
It will pixelate as it compresses, and the type is blurry depending on how big the piece is and how much content there is. You may try creating a cover eblast piece in a small piece that has little verbiage, and then link to the newsletter or attach the newsletter to the email. But an 11 x17 piece (for example) with some art and text which I tried is just messy to the eye, even when I increase the view after it is received.
If you plan on emailing more than 25 recipients at a time, you will most likely need to use an email service.
Emailing an image that makes up the entire message is not effective at all.
If you use images in an eblast, they really need to be saved and optimized for web from Photoshop.
While a newsletter for an eblast can be created in Indesign, it really needs to be constructed as html and using the proper tools are recommended.
There's an AppleScript that will convert your InDesign file to Photoshop, which you can then use as basis to code professionally looking email.
As folks commented earlier, InDesign is not the right tool for anything web-related beyond early mockups. The HTML export feature is a joke, it will not create clean, cross-browser code, and that's the case with any software product used to generate web layouts of any serious complexity.