That is not a belief I wish to perpetuate. Sendmail and Fetchmail are popular examples. Reading it is easy. You can make it work, but it will be painful. I cannot get sendmail to run.
All mail is delivered through that central host.
No local delivery is possible. Most other Sendmail features are disabled. However, you need to understand what they are doing. See above about a fully independent mail program. The other three configuration lines are easier to explain.