For unsolicited submissions, most literary agents prefer email queries with sample pages pasted into the body of the message. Email submissions save time and trees, but if sample pages are requested with a submission, it’s better to ask for that material as an attachment. Here’s why:
An attached file is the only way to maintain the integrity of the submitting author’s work. Emails are sent and received via one of three formats: HTML, rich text, or text only. When the email formats don’t match, the sender’s email loses its intended styling (and Gmail restyles everything, grrr!). Pretend George R.R. Martin doesn’t have an agent, and he queries you, but your email program renders his sample material completely awkward because it eliminates his use of italics to express first-person thoughts. In evaluating Martin’s submission, you would assume that he doesn’t know the difference between first- and third-person narration. Oops. Good writers format their work appropriately, so why let an email program ruin a potentially great query by butchering its format?