Thank you for the reply.
Maybe with my question, I didn't give you understand completely my concern on mail servers understanding.
So, last days, I have read a lot of web pages, blogs, articles on them, I have seen many many YouTube videos, trying to understand the following:
There are two kind of servers, as far as it concerns mails:
a. Web SMTP mail server (for a domain)
b. local installed mail server
What exactly is [b.] ?
Many many terms on mail environments. And specifically, on mail servers!!!
Also, on game, into play, there are some companies called Email Service Providers (ESP). These ESPs, have nothing to do with all that? e.g. Google SMTP mail server, or Yahoo! SMTP mails server, etc, where and what is their ESPs???
Too many question on my whole concern.
But, my main question is for the Web SMTP Mail Servers vs local mail servers (MS Exchange, Postfix, etc.).
So, since there are the Web SMTP mail servers, that will do the whole job of delivering email messages, why there are also these local email servers to install? What is the difference between them?
That is the main question I cannot understand.