Thanks for clarifying all but one question because I think I was not able to explain that question properly.
Why not use smtp instead of mail function?
Now you are assuming that I am using the same server to send email using SMTP but normally SMTP is used on other server as well.
Let me give you an example.
Say I want to send emails from xyz.com and use external smtp server provider (Say Google) to send my emails and that way it does not matter if my xyz.com has mail enabled or not.
You are actually trying to re-invent the SMTP system with PHP mail function. Why not use SMTP of Google from reliable host1 instead of using it through a chain of hosts which are anyway unreliable.
Also one more fresh question.
What happens when host2 suddenly stops the mail function after understand it is being spammed.