The following excerpt is from $uccess, Your Path to a Successful Book by Maralyn D. Hill and Brenda C. Hill.

Now comes the challenge–the exceptions to the rules.
Some small publishers and magazine publishers have different guidelines.
We were sending numerous articles to one magazine that requested single spaced and just a hard return at the end of the paragraph. When we sent something to another magazine, we used the same formatting—wrong move, and major faux pas.
We sent a letter of apology and resubmitted, but this is not something we want to experience in the future.
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Maralyn D. Hill and Brenda C. Hill