Use external Mail Server in WordPress Website – even several servers in one

If we want to use external mail servers for sending mails via our website, we can do it without any plugin.
And also we can build simple function which can use several mail servers for each case.
For example, it may use different mail server when it sends something via cron jobs, another mail server when it send something via contact form etc..
It is not about to change email headers like “email from”, “sender name” – to do it is much easier.
I mean real external mail server with different mail server hostname, port, user and password.
 1. In first one it gives a code to use single external mail server.
2. In second one it gives ability using randomly chosen multiple mail servers. (could be helpful when mailservers have some sending limits)
3. In third one it gives ability chosing mail servers for specified cases.