Microsoft Exchange

I’m presently hosting websites for some small businesses. They are currently using Hotmail/yahoo to receive their email. I want to provide email service for my customers. Can Microsoft Exchange be used as an internet email server? I would like the acct’s to be setup online through a forms page and Exchange creates the acct. I was told I need to purchase an email server software that does all that and because Exchange can’t. If you can provide a book with information on how to setup exchange as an internet email server I would greatly appreciate it. Thank you