What SMTP architecture and software is used in big reliable notification systems?

I wonder what software is used in project depends on tones of SMTP notifications? There probably are sev开发者_开发技巧eral servers so what is used to ensure sender authentication success (maby additional proxy servers)?

Outsourcing this problem is easy so please don't consider that.

Not sure what you mean by "tones of SMTP Notifications". You mean emails sent over SMTP, right?

There are many reliable mail systems you can use ranging from Exchange (requires Windows and Active Directory) to SendMail, Postfix, qMail and probably a bunch that I don't know of. All of these are enterprise worthy stable systems capable of handling very large amounts of mails.

In addition there are several smaller systems that are probably reliable.