After all, your tenth-grade typing teacher drilled it into your head that two spaces were proper. This was because all of the characters on a typewriter had the same amount of space allotted to them. In professional publishing, and later, with the advent of computer fonts, letters were given proportional amounts of space.
More space looks awkward and disrupts reading in a professional publication.
I have heard the argument that, in the case of e-mail text, that two spaces are permissible. I personally don't agree. It's been such a struggle to get most of us to remember to use one space rather than two that creating an exception seems to be asking for a step backwards. Also, keep in mind that e-mail programs use the fonts in your computer system which are proportional fonts anyway. If you disagree with the need for one space or two, here's an additional thought.