There is a sort of folk myth that two spaces should be used after punctuation. As far as I know this has never been the case, and as a proof reader I can tell you that multiple spaces have the potential to play merry hell with word processors under certain circumstances, especially when set to block justify.
The myth probably started with mechanical typewriters, which by necessity used fixed-width typefaces, meaning that a period was the same width as an "m". Because the actual period is just a small dot, this can give the illusion of a double space.
On a related note, leaving a blank line between paragraphs is also strictly incorrect, although when writing e-mails and message board posts you have no alternative. Strictly speaking, you should create a stylesheet which spaces the paragraphs apart and only press the Return key once between paragraphs. This does create some complicated setting up procedures and probably way too much trouble for a simple letter; but for really professional publications, you'd be doing translators and proof-readers a great service by doing it properly (apart from other considerations, blank lines can confuse word processors when it comes to inserting automatic page breaks).
Even worse is the practice of using multiple spaces to indent lines or tabulate data. Please use proper margins for indentation and tab stops or actual tables for tabulation.