From an agent:
Understand that if someone takes email queries, you're better off NOT just cutting and pasting your printed query letter into an email and hitting send. You have to write to a different rhythm.
People read differently on the screen. Actually move their eyes differently, in fact.
Short paragraphs, short sentences, lots of white space.
You can't double space between each line to create white space, so you have to construct your sentences differently AND lay them out differently on the page.