If you are closing an email by thanking someone, "Thanks for your help" would be appropriate in an informal email. What would be suitable in a formal email?
-Thank you for your help -Thank you for your assistance in this matter -We appreciate your help in this matter
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 ask 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 you were writing to someone about an upcoming meeting, why would "Hello" be a bad subject line (title)?
It doesn't explain the reason you are writing me or who you are. A more specific subject line makes sure that the message isn't mistaken for spam, or simply ignored. for example, Re: 10:30 AM meeting next Wed.