When writing fiction, we generally use past tense for the narration and present tense for dialogue. When you are done with a fiction novel and are ready to query a literary agent/publisher, what should the tense be for your synopsis, outline, query letter?
I assume that when you ask about the grammatical tense of your query letter you are asking about those portions of your letter that recount the story of your novel.
In a summary, outline, logline, or other synopsis of a narrative the convention is to use present tense. Whereas a novel might begin: "In a hole in the ground there lived a hobbit.", a summary of that novel will usually present the events in present tense: "The story begins in the Shire, where the hobbit Frodo Baggins inherits the Ring from Bilbo Baggins, his cousin and guardian."
The reason for this, I believe, is that we are not, when we summarize or outline a narrative, any longer narrating a story, but rather we present its narrative structure, that is we present the elements of the narrative, and those elements are not temporal. For example, "Frodo inherits the Ring" is a structural element (that could take place in the past, present, or future of the narrator), while "Frodo inherited the Ring" is a narration of events that took place in the past (in relation to the narrator). You can think of the outline or summary as a typology of your novel in which you present the motifs of which your novel is composed.
The other parts of your query letter should be in the tense that correctly represents the chronological relation of (e.g. "I have already published three novels, and today I work as a full time writer.").
Present tense, because they are all dealing with what your work IS.
Your query is a short passage, just a few sentences, trying to sell your story to the agent.
Your outline and synopsis are both longer and go further into detail on what your story is, and what it's telling about.
Think of it as you presenting the book to someone you've never met. You're telling them what it is and why the should read it/buy it/sell it etc. You don't attract attention by telling them what it was.
Edit: I want to expand on this some more, as my initial response was done with little time. Essentially, your query, synopsis and outline are all business propositions, and should be presented and written as such.