Standard practice is to negotiate the terms of a contract, once both parties are happy with the terms, the contract is then signed by all parties.
How is sending an email saying 'yeah those terms sound okay, send through the contract to be reviewed and (possibly) signed' suddenly become a legally binding contract.
I go into a caryard and ask how much for a car including all 'onroad costs'. I get a price I think is acceptable, I say I'd like to buy it and the salesman says I'll get the paperwork ready. Am I now legally obliged to buy it?
I was offered a job, I said yes I'd like to take the job. The company then went and got a contract written up and sent through to me. I read the contract, and as I wasn't happy with it, I advised the company I was no longer interested (after trying to negotiate). Was I legally obliged to take the job?