Your e-mail address. If people don't know who you are, don't answer to you, or don't think of you as a source of information/work/ideas, then you're immediately lower priority.
O.K., so what's the second most important thing? The subject line. Scott the Nametag Guy explains why:
Because that’s the only thing customers see. Because that’s the only thing customers have the time to read. Because that’s the only thing customers will use to decide whether or not to open it. Because that’s the only way you can immediately differentiate yourself in their inbox.If your e-mail address or subject line doesn't give the reader a sense of urgency, your message may never get opened. So if you want to ensure that it does, read the rest of Scott's post.