single sheet, list of transactions- in and out. How to identify incomplete transactions?

I have asingle worksheet of over 8000 lines. They are all transaction that are identified by an email address. Every 'in' shoudl have an associated 'out' on the same email address. How do I run a quesry (is that the bestway..?) to identify the 'in' transactions that dont have a corresponding 'out' transaction?

Yes, you guessed, I am almost complete novice to using excel other than for adding up and taking away long lists of numbers.

Any help. suggestions, tutorials, previous posts much appreciated.

Thanks for your time.

just realised there is a sub forum for new users/basics. Will move post there...