[X-Unix] Beginners questions on Unix Mail

[kim]

I am a complete beginner in Unix and I am trying to understand how to 
send and receive email.

First off, where does the Hostname come from? By that I mean, how is it 
generated? Mine begins with the IP address, is followed by a bit I 
don't understand, and ends with the domain name of the Internet Service 
Provider.

The start and end make sense to me, but where does that middle bit come 
from? Does it mean anything in particular? And can I change it?