We send e-mails so casually and with such informality, even in the business environment, that it is easy to forget that they may carry significant legal consequences. It is only prudent to bear in mind that even e-mails written in the most conversational style may create legal obligations no less binding than a more conventional written agreement laden with legalese and signed with all formalities.
If a business wants to entirely avoid the possibility of having e-mails treated as binding amendments to existing contracts, the best approach is to be as clear and direct as possible on the subject by including language in contracts to the effect that e-mails do not count as signed writings for purposes of any contract amendments.