In light of Postfix versus Exim , you might wonder whatfeatures make Exim into a mailer construction kit. For me, the easiestway to summarize the answer is to say that Exim has the idea of whatI will call a user-written mail processing pipeline (actually two ofthem, sort of).

By a mail processing pipeline I mean a series of steps that messagesgo through to decide what will happen to them and how they will bedelivered. In many MTAs, this processing pipeline is more or less

Once upon a time, the Internet was just something that you used tocommunicate with other universities (and companies). One consequenceof this was that the university needed to provide everything for itsown people; all of the services they needed needed to come from theuniversity.

This is no longer the case for universities. Increasingly, people nolonger want you to be their service provider ( partly because theyalready have their own ), and on top ofthat other people can do bits of it better than you can