Postfix expert needed for dialup config

[Tommy Trussell]

As I said yesterday I got postfix email to flush properly, but last
night I realized the LOCAL mail is not being routed correctly... If I
use the sendmail command at a terminal prompt to try to send a message
to root, it tacks the ISP's domain on it and queues it for SMTP
delivery -- root at fastmail.fm -- NOT what I want!

So yesterday I added the canonical_maps and virtual_maps settings and
created the db files in the postfix directory, but they have had NO
effect yet...

It occurs to me that I haven't tried everything -- maybe I have to
include all possible permutations, such as root at localhost,
root at fastmail.fm root at localhost.localdomain ?  Maybe I'm putting the
items in the wrong order in the canonical and virtual data files?

Is there an EASY setting I'm missing that will tell postfix to leave
local addresses alone? What have I done to mess up local delivery?

I only need this because the system is configured to report errors via
email, but I sure don't want those messages to go to
random-username at fastmail.fm !  Hmm... how do I configure error message
delivery? Is that in base-config?

The people who advised me to just teach the user to use webmail are
beginning to look very wise! If I can't get it figured out I'll undo
my settings and dpkg-reconfigure postfix and hope that will fix the
local delivery.