There is a lot of information on this because sadly, it is not uncommon. The last word syndrome is a well-known psychological flaw. It frequently manifests itself in a person whose conversations, written and spoken words expose an egocentricity and need for supremacy. When any feeling of power is taken away from them, it irks them so much that they always look for ways to be the one to finish the conversation.
People with this bitter malediction usually strive to be the only expert voice, in their chosen field, over the people around them . They always know better. This feeling of supremacy naturally leads to a lack of respect for others that they are engaged with. It comes through with sarcastic and often childish attacks in their speech or writing. It is a form of bullying. However, while they feel superior, they often never rise to the top in their career or business due to their real innate and deep-down insecurity.
A person who uses this form of power-play is often a very competitive, hardheaded person, possibly a high-level sportsperson from an individual sport such as squash, tennis or boxing where the last shot is always meant to be the winning shot. In their new domain of the business world, Email is their new game and the ‘last word’ their weapon of choice.
The only way to deal with this kind of person is to let them have the last word. Let them feel superior and let them revel in their churlish behavior because it will never change and you are often, unsuspectingly, just playing their game.