I work in professional publishing. Europe has never, ever used double spacing. Typewriters don’t need this, it was considered clearer to the eye because of the obligatory single-space fonts they use, but it was typically an US cosmetic fashion.
@JohnAtl You might be confusing the space between sentences and between lines. Double-space intervals have their editorial uses, but they are not a typographical matter. 
And I second all that everyone else said in that thread: keep email formatting to the bare minimum if you want to make sure it will be received correctly.
For signatures, there’s an app on the MAS called Email Signature Creator which provides beautiful rich formatting while remaining portable. I haven’t had a problem with it in two years of use.