Goodness, but e-mails have changed things mightily from when I started out. You can get rejections just THAT much faster!
I'd suggest going over the agent's website (or that Writer's Market entry) very carefully to check on the rules. So many are picky about accepting only snail mail queries vs e-queries.
If, after a very careful check over so you know you didn't miss it, you can't find specifics, it's ok to e-mail and ask what specifics they want with a query letter.
Sometimes they will inform you they're not accepting queries because they're presently flooded and just can't look at anything else. Some of the big publishers will have full schedules for 2-3 years in advance.
I sent my first query to Murder Ink back in the day. It wasn't much of a letter, but they wrote back in my SASE and asked for the 3 sample chapters and outline. I thought the initial query was a waste of time, since it didn't tell them much, but all the books I'd read on how to get published said to go through the hoops even if they don't make a lot of sense.
Oh--I didn't sell to Murder Ink, but the process did get my feet wet. The next time around I was a bit more confident!
(And the next and the next and the next and the next and the next....)