Add basic docs for Postfix w/ Gmail relay

docs/base/email/postfix.rst

@@ -1,5 +1,43 @@
 
+.. highlight:: sh
+
 Postfix Setup
 =============
 
-TODO
+The typical scenario for a Rattail server is that it runs Linux and
+Postfix for the mail service.  Here we describe that typical setup.
+
+
+Basics
+------
+
+If you don't already have Postfix installed, do that first::
+
+   sudo apt install postfix
+
+It's normally a good idea to declare your domain and hostname.
+Assuming your server is ``myserver.example.com`` then::
+
+   sudo postconf -e 'myhostname=myserver.example.com'
+   sudo postconf -e 'myorigin=example.com'
+   sudo systemctl restart postfix
+
+
+Gmail Relay
+-----------
+
+Make sure Postfix knows to send all mail through Gmail relay::
+
+   sudo postconf -e 'relayhost=smtp-relay.gmail.com:25'
+   sudo systemctl restart postfix
+
+This however assumes the Gmail relay is also configured to allow mail
+from your server.  The most straightforward way to do this is
+via IP address authentication.
+
+See `Google Help`_ for more info, but the gist is that you should
+choose "Only accept mail from the specified IP addresses" for the
+relay authentication within Google Admin settings.  And then specify
+the public-facing IP address for your server.
+
+.. _Google Help: https://support.google.com/a/answer/2956491?hl=en