echo root | mail root
echo user1 | mail user1

# /etc/aliases
mailer-daemon: postmaster
postmaster: root
nobody: root
hostmaster: root
usenet: root
news: root
webmaster: root
www: root
ftp: root
abuse: root
noc: root
security: root
root: user1
user1: my_email@gmail.com

# This is /etc/email-addresses. It is part of the exim package
#
# This file contains email addresses to use for outgoing mail. Any local
# part not in here will be qualified by the system domain as normal.
#
# It should contain lines of the form:
#
#user: someone@isp.com
user1: my_email@gmail.com

The response was:
DNS Error: 42317832 DNS type 'mx' lookup of hostname responded with code NXDOMAIN Domain name not found: hostname


Final-Recipient: rfc822; user1@hostname
Action: failed
Status: 4.0.0
Diagnostic-Code: smtp; DNS Error: 42317832 DNS type 'mx' lookup of hostname responded with code NXDOMAIN
 Domain name not found: hostname
Last-Attempt-Date: Sat, 22 Jul 2017 21:49:10 -0700 (PDT)


---------- Forwarded message ----------
From: my_email@gmail.com
To: <user1@hostname>
Cc:
Bcc:
Date: Sun, 23 Jul 2017 00:49:09 -0400
Subject:
user1@hostname