Saving a File with Illustrator
The last step is often overlooked. Arguably the Best File Type for email, and for customers to view is a .PDF. The reasons .PDF is the best are that 95%* of computers have Adobe Reader installed, as well as .PDF can often be viewed directy in an Email Client or Web Browser. They are also compressable, can be passworded (dogme95) and typically have a low native file size.
Once you have your file the way you like it, simply go File < Save As... Navigate to where you would like to save the file, and select Adobe PDF (pdf) from the Format Menu.
This will bring up a menu with tons of options, but typically the [Illustrator Default] will work great as long as your file isn't super Complex or have lots of Links, or Raster Elements.
If the file size is to large to email >10mb for most emails, but sometimes >5mb. If this is the case you can select the [Smallest File Size] from the Adobe PDF Preset: menu. This will create a compressed version that will be better to email. It is best to save this as a different name because if you compress the file it is no longer easily editable to save file size. I typically just add a space after the file name and save a Native version, and Compressed version.