Jpegs are already compressed. "Raw" image files are compressed to a degree as
well but the compression is far less than for jpeg so all the original image
data is retained. Jpegs lose a lot of the original image data - and that's why
camera companies call jpeg a lossy picture file format.

The only way to make a jpeg file smaller is to reduce the image size. For
example, if your camera produces images that are 3872x2592 pixels, reducing that
to 1280x858 will considerably reduce the size of the file while retaining an
image that'll look fine on the screen. The reduced image size will make getting
larger printed images problematic.

So, depending on the intended use of the e-mailed images you may or may not want
to reduce the image (and thereby file) size.