You have patterns you repeat when answering emails and otherwise messaging. These are similar to other peoples but have your style of expression. Today you can already automate these on many devices by using snippets and a shortcut. The only problem is that they are always exact copies, so answering the same person twice will reveal this fact.
A smart snippet software that, while you are typing, analyzes your words and suggests an auto-completion of the whole sentence and not just one word. It will do so with a slightly different variation based on how you started your sentence and your writing style. As you continue writing what it suggests changes until at some point, you choose to use it and complete the sentence or even a whole paragraph.