The phrase ‘unable to accept unsolicited manuscripts’ or something similar is a common refrain from many large publishing houses as well as some agents. This is for good reason, and can vary, but often helps keep the volume of manuscripts received manageable; hopefully it also means solicited manuscripts are of a publishable caliber.
A writer represented by an agent can often get their manuscript in front of a senior editor at a publishing house with this rule in place because of the professional relationship the agent and the editor already have.
Meeting an editor at a workshop or conference is a great way to get to know that editor, their pet peeves and their potential ‘wish list’ of books they’d love to see. Many editors also give attendees the opportunity to send a query and partial manuscript their way as long as its complete and polished.
Take the editor’s ‘Do’s and Don’t’ to heart and use them to ensure your manuscript is something they’d consider. Take the time to polish and revise your manuscript before sending it out. And write a professional query letter with a great hook to whet the editor’s appetite so that she might ask to see more of your manuscript.