When in doubt, do nowt.
Most of the time you can just flow straight into the email.When a greeting is required, usually just saying "Hi" and ending with "regards" will suffice as this is quite commonplace.
Other than that it depends on your relationship with this person. If you have never talked to them then you should always err on the side of caution and be formal and say Mr or Mrs/Miss/Ms.. so that would be
"Hi Mrs Benjamin." if you don't know her, or "Hi Flo" if you do.
Why act in these ways?
The email is already addressed to the recipient. Each email sent, one to one, the recipient is automatically assigned to be the reader. This is the departure from hand written letters. It is obvious that the email is from the sender and the recipient is a given.
This is one of the unique features of emails being immediate asynchronous communication that is personal via a personal email account. A greeting is used as a vehicle to flow into the actual content of the email.
The actual statement being made, the headers and footers (hi, regards) are just a way of starting and ending the email, and provided these are polite there isn't much thought given to them.