Though email isn't as formal as a hard-copy business letter, it's also not as casual as chatting with friends. Even though email's been around for years, the etiquette for using it properly isn't always clear. To set the right tone for the email you're sending, you need the right greeting.
In business email, it's better to err on the side of formality. With someone you haven't emailed before, use his formal title -- "Dear Dr. Smith" or "Dear Ms. Jones," for instance. "Dear" is much less used in email greetings than it used to be, but etiquette experts still recommend it for an initial business contact. If the recipient doesn't know you at all, follow the greeting with an introduction -- who you are and why you're contacting her.
If the other party greets you in email by your first name, or asks you to use his first name, you're safely on a first-name basis. Reserve a casual opening such as "hi" for colleagues you know really well and are comfortable with. Even in a casual email, be certain that you have the other party's name spelled correctly. If you send multiple emails in a single thread, only the first one or two need a salutation.
If you send a message out to multiple people, the first email still needs a salutation -- "Dear Tom, Dick and Mary," for instance. If you have too many recipients to list all their names, "Dear Team" is a practical alternative. As with individual recipients, if you're emailing back and forth, you can stop using salutations after the first letter.
You have a lot more flexibility when writing a personal email. The first name or a simple "hi" or "hello" are fine, unless you know the recipient has a strong preference. A casual email can safely ditch "dear," as it often sounds stilted. Even in an intimate email, "dear" can sound more like a formality than a sincere expression. Using the person's name or something more extravagant, such as "my dearest," may work better.
Along with your greeting, pay attention to your subject line. It's the first thing the recipient's going to see, so you want her to know what you're writing her about. Never leave it blank or use a misleading subject line to get attention. Definitely never put "Urgent" in it if the issue isn't really urgent.