I recently received an email in which the format attracted my attention more so than the content itself. My eyes were instantly drawn to the white spaces and I couldn't stop looking for them as I began to read. Each sentence felt isolated, rather than being part of a whole communication.
It originally used to be common practice when using typewriters to leave quite a significant gap after each sentence to aid legibility. This was however when the type itself was monospaced, meaning that each character had a fixed width and occupied the same space horizontally.
Since the introduction of proportional fonts and the ability to adjust kerning we are able to produce much more visibly pleasing and more legible text, leaving the double space redundant.
So don't let a double space invade your next email.