I must admit I don't understand what greeting I should use on a query addressed to Submissions without breaking the use-the-correct-name rule. Do I go with "Attn: Representatives of the Big-Deal Literary Agency"? "Dear people"?How about "Hey, you guys"?
For any letter, in which you do not know the name/ gender of the person to whom you are writing, you may properly use: "Ladies/Gentlemen:" even if it's only one person.
If you know the person's name but NOT the gender, I use first person thus: Flotsam Jetson is "Dear Flotsam:"
And in a pinch, I've been known to use "Good morning"
Source of all etiquette advice: Grandmother Snark