I'm always emailing my tutors about something or other, but we call them by their first names so we lack the formality in emails... My tutor once used the phrase 'some IT tw*t has deleted the file' which made me giggle.
How about:
'Morning!/Hello,
...
Thanks a lot,
Ella S'
then when you're replying just miss the 'dear' section as if carrying on a conversation.