Definition
Plain language
A widely used program that runs email servers, handling how messages get routed and delivered.
As stated in the literature
An open-source mail transfer agent; used as a computer-use and world-model case study where correctly predicting that the server validates unknown recipients before consulting routing determined whether an agent applied the right fix.
Why it matters: It quietly moves much of the world's email, so understanding how it routes messages matters for anyone diagnosing why mail fails to deliver.
For example, when you send an email to a coworker, software like Postfix decides where the message goes and hands it off toward their inbox.
Heard on the show
“Both agents — before warm-up and after — try to configure a Postfix mail server, and both hit the same wall: a recipient gets rejected.”Episode 167 — How Teaching an AI to Predict, Not Act, Made It a Better Actor