What email server software do they use (myspace, facebook, etc) to send so many emails?

Do they use a common server open source software like sendmail, exim, or do they custom build their own? How do they send so many emails?

What email server software do they use (myspace, facebook, etc) to send so many emails?
all of what you listed will do the job. sendmail/exim/qmail are all just transportation programs. The real tool is mailing list software. Its what it actually is, it saves a list, you write the email, and the transport software will run through the list and send it out to everyone.