If possible, find out the name of one of the people in the admissions office. Sometimes different admission staff handle different departments or different letters of the alphabet. If the name is found, write Mr. (Last Name) to begin. For women, it is a bit trickier because there are different salutations depending on marriage status. In those cases it may be better to address the email using the first and last name. If it is not possible to find a person's name within admissions, then a generic "To Whom It May Concern:" will suffice.
Always check the email for typos and grammatical errors. Nothing looks worse than a poorly written email, and it may give the admissions office a slightly negative view toward the person and their application. If a person's email server does not have a quality spell check function, type the email first in a Microsoft Word document so it is easier to catch mistakes. This is also a good tactic when working on a computer that has the tendency to do unpredictable things such as freeze up every once in a while; when this happens, an email could be lost before it is saved.
Use proper spelling. Do not use text message abbreviations or other slang. Those are appropriate for friends and family, not for more professional interactions. For example, don't write "I am interested in learning more about ur school;" rather, spell out "your" correctly. When emailing the admissions office it is always a good idea to give a positive impression with your communication skills.
Be straightforward in the email. Don't tell a long and drawn-out story about how badly you want to be admitted or why you didn't get an application form in on time. Summarize what you want to convey in a short manner so that the admissions counselor does not see one or two good-sized paragraphs and have to take the time to read through information they don't need.
Always end the correspondence with something like "Sincerely," or "Best Regards" before typing your name. This continues the professionalism trend and shows that you appreciate admissions taking the time to read the email.
Lastly, type in an appropriate subject line heading. It needs to convey what the email is about. Sometimes wording the subject line heading in a unique or interesting way can catch an admissions counselor's eye, and they might read the email sooner.