How do I send my Mathematica document by email?

Attach it

Mathematica notebooks are saved as pure 7 bit ASCII, so you can attach the whole notebook to your email message without fearing to loose any information, like typesetting or graphics. This is the preferrred option for long notebooks or messages that are likely to be forwarded.

Paste it

There are multiple options other than sending attachments to include all or part of your notebook to an email message. The best option is to copy and paste Notebook expressions or complete notebooks into your email. To do this, select the pieces you want to add to your email and select Copy As...Notebook Expression or Copy As...Complete Notebook. In both cases, the addressee can just copy the expression out of the email and paste it into Mathematica. Mathematica will automatically recognize the code and create and render it in a new notebook. 

Copy As...Notebook Expression returns the command Mathematica would use to generate a notebook that includes just the selected cells. It results in a somewhat smaller document, but the adrressee must have a copy of Mathematica to paste into to be able to see the rendered notebook.
 
Copy As...Complete Notebook returns a complete notebook, including just the selected cells as it would be saved on your computer. This results in a longer document but it can be saved on a computer and opened with MathReader or Publicon.

Be careful if the notebook or expression you are sending is likely to be forwarded. Most email programs insert forwarding charactes like ">", tabs or vertical lines. These result in systax errors when Mathematica tries to render the notebook and have to be stripped before pasting into Mathematica.