If you were writing your instructor (me) an informal email, you could start with just "Julia,". If you hadn't met me before but wanted to email me, what would be the best opening?
Dear ----
If you are starting a new email "conversation", you usually start by explaining the purpose of the email. Informally, you could start by saying: -I'm writing about. . . - I just wanted to talk to you about. . . - I just wanted to ask you about. . . -I just wanted to tell you about. . . How would you do this in a more formal email?
-I am writing in connection with. . . -I am writing with regard to. . .
If this is not the first email, phone call, or message of the "conversation", it is a good idea to refer to the previous contact. Informally, you might say: -Re: your last email. . . -Thanks for your (last) email. How would you do this in a more formal email?
-With regard to your email of (date),. . . -Further to your email of (date),. . . -In reply to your email of (date),. . .
If you were writing me to ask an upcoming lesson, why would "Hello" or "Hello Julia" be a bad subject line (title)?
It doesn't explain the reason you are writing me or who you are. If you are writing an instructor, the name and time of the class would be appropriate. For example: Re: 10:30 AM English Class next Wed. Also, a more specific subject line makes sure that the message isn't mistaken for spam, or simply ignored.
If you don't know the name of the person that you are writing to, you can't start with, "Dear Mr./Mrs./Miss/Ms. *****". How would you begin instead?
-To whom it may concern -Dear (Corporate Finance/Accounting/Marketing. . .) Team -Dear Sir or Madam