This was a real question I was asked by a non-technical person. The answer is nothing that you could put inside a technical book, but maybe will help you to better understand how things work.
what does "relayed" mean (when talking about email)?
in a relay race, contenders in a team hand on the baton to each other until they carry it to the finishing line.
When you send an email, you hand it on to a server that takes it and hands it on to another server that knows how to deliver it to the final destination. This process can happen again and again in a chain of servers, until the message reaches its final destination.
This process of handing the message on from a server to the other, so similar to a relay race, is also called relay.
Depending on the conditions, a server may allow or deny the relay. For example, I can relay mail using Tiscali's mail servers when I am on their network (e.g. using my Tiscali ADSL at home), but when I am in Oslo, I cannot.