Just about any and every server needs to be able to send email - whether it's end-user-email, like password recovery services for a website to emails to system administrators reporting on the status of system backups and errors. The problem is that it's non trivial (understatement) to set up a mail server properly.
free & open source
The Open Education Resource universitas (OERu) is an open organisation from top to bottom. Our entire technological infrastructure (with a couple exceptions) is built with and on Free and Open Source Software to which I normally refer as "FOSS". This introductory post outlines our technological motivations and some of our key choices. Future blog posts will go into more depth on some of these choices, and provide more insight in to specific implementations.