Final answer:
Part [1] of an email is the subject line, which is essential for indicating the main topic of the email and influencing whether the reader will open it.
Step-by-step explanation:
Part [1] of the email, as described in the question, refers to the subject line. The subject line is crucial because it lets the reader know what the email is about. This part of the email is comparable to an essay title and should succinctly represent the main idea or topic. Good subject lines are clear and specific, making it easier for the recipient to prioritize and understand the content of the email. Moreover, a well-crafted subject line can influence the recipient's decision to open and read the email, especially if they are a busy person. It should be specific enough for easy reference at a later time.