A cover letter is considered to be a formal type of business correspondence. Therefore, your greeting should start with “Dear ____,” just as you learned in school. If you aren’t submitting your cover letter to a specific person, we recommend “Dear Hiring Manager.” But in an ideal world, you’ll have figured out the hiring manager’s name and can use “Dear [first name].” If last names are more your style, that’s fine, but if the hiring manager emails you back and signs her first name, consider yourself on a first name basis from then on. And when closing your cover letter, “Sincerely,” is always a safe bet.
Because email is a more casual form of correspondence, it’s even more important to figure out the hiring manager’s name in a cover email than it is for a cover letter. If you MUST apply to a generic email address, you can write “Dear Hiring Manager,” but if you’re able to figure out the person’s name, you should use “Hi [first name].” Why “Hi” and not “Dear?” This less formal greeting will make the reader feel more comfortable with you -- most people don’t start their regular business emails with “Dear _____,” so why would you do it in a cover email? You want to go for a personal feel to give your cover email maximum effect. If you want to end your cover email with a more formal sign off, you can always use “Sincerely,” or “Best,” but a simple “Thank you!” can also go a long way.