You might think having an agent come back to you with a request for your work is the next best thing to your own version of absolute heaven (books, wine, chocolates, Angelina Jolie or Brad Pitt or a Caribbean holiday or whatever else represents sublime sensory experience for you).
The very first agent I queried came back to me in less than twenty four hours and I emailed back, in all my idiotic inexperience, with the same subject header and a file attachment. Nothing wrong with that, is there? After all, that's how you keep track of email conversations, isn't it?
No. First of all there are a set of rules and the first one (after snagging an agent's attention) is to let them know you are no longer querying but have been elevated to the status of submitting requested material. How do you do that? NOT by simply hitting 'reply' and attaching a file. You change your subject header to 'Requested Materials - BOOK TITLE', attach the file, and then hit 'send'.
Said agent always responds to submitters of requested material. Said agent posted on said agent's blog almost two weeks ago that said agent has just about finished reading submitted material. Said agent posted this week that queries are closed and said agent is proud of nine (NINE!!!) new clients. Said agent is still pining for the elusive middle grade high concept fantasy (THAT'S ME THAT'S ME THAT'S ME).
Said agent's email address for queries is now on auto respond - said agent is closed til January of next year.
Moral to the story: Finish your manuscript. Spend time on forums learning how to query an agent. Query agent. Spend more time on forums learning how to respond to requests for material before sending the requests. And pray after you send your requested material that an internet goblin doesn't hex it out of existence before the avid agent's eyes alight on it, because woe betide you if you email avid agent asking if avid agent has received the material. That's akin to labelling yourself rabid internet stalker.