One or both friends may unload a lot of frustration in their emails during this type of exchange, and in doing so fail to listen to the other side. The result is that the problem (and level of anger and hurt) escalates. With an email fight, people accuse the other person of doing things without allowing for that person to explain, which further complicates an argument. True resolution comes when both sides are willing to listen and work though their issues.
More about resolving conflicts with friends:
"I wanted to mention how hurt I was when my friend ignored me at the party, but she denied it and then we got into a huge email fight. Now, we're not even talking."