When working with Ubuntu servers, many administrators encounter an issue where cron job outputs and system mail intended for root@localhost mysteriously get routed through the domain's primary mail server. The core issue lies in Postfix's configuration interpreting root@mydomain.com as an external address rather than applying local alias rules.

From the mail.log snippet, we can observe the exact routing behavior:

Mar  7 09:39:18 linux1 postfix/smtp[31558]: F3B9C98025E: to=<root@mydomain.com>, 
orig_to=<root>, relay=my.isps.relayhost.com[<IP address omitted>]:25, 
status=sent (250 Ok: queued as A97F5D8126)

The key indicators are:

• Postfix is rewriting root@localhost to root@mydomain.com
• The message gets relayed through ISP's SMTP server
• Local aliases (/etc/aliases) are being ignored

Three configuration files significantly impact this behavior:

1. /etc/postfix/main.cf - The core issue stems from these settings:

myorigin = /etc/mailname
mydestination = linux1.mydomain.com, localhost.linux1.mydomain.com, localhost
relayhost = my.isps.relayhost.com

2. /etc/mailname - Contains just the domain:

mydomain.com

3. /etc/hosts - Has unexpected VM hostname entries:

127.0.0.1 localhost
127.0.1.1 linux1.mylinux.mydomain.com linux1

Step 1: Correct Local Delivery Domain

Modify /etc/postfix/main.cf to ensure proper local delivery:

# Change myorigin to use system hostname
myorigin = $myhostname

# Ensure localhost is properly defined in mydestination
mydestination = $myhostname, localhost.$mydomain, localhost

# Optional: Explicitly define domain for local mail
mydomain = mydomain.com
local_recipient_maps = proxy:unix:passwd.byname $alias_maps

Step 2: Verify and Update Aliases

Ensure /etc/aliases contains:

root: anothermail@anotherdomain.com
postmaster: root

Then rebuild aliases database:

sudo newaliases
sudo postalias /etc/aliases

Step 3: Test Email Routing

Use these commands to verify the configuration:

# Test local delivery
echo "Test Message" | mail -s "Local Delivery Test" root

# Check mail queue
postqueue -p

# Verify logs
tail -f /var/log/mail.log

For more complex forwarding scenarios, create /etc/postfix/virtual:

root@localhost anothermail@anotherdomain.com
root@linux1 anothermail@anotherdomain.com
@localhost anothermail@anotherdomain.com

Then add to main.cf:

virtual_alias_maps = hash:/etc/postfix/virtual

Compile the map:

sudo postmap /etc/postfix/virtual
sudo systemctl reload postfix

• Always check /var/log/mail.log after configuration changes
• Use postconf -n to verify active Postfix settings
• Test with sendmail -v root < /dev/null for detailed delivery info
• Consider SELinux/apparmor permissions if issues persist

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Many sysadmins face this exact issue where cron jobs send root's mail to an unexpected external address instead of handling it locally or forwarding to a designated administrator email. Let's examine why this happens and how to properly configure Postfix.

From your mail.log, we can see the mail flow:

Mar  7 09:39:17 linux1 postfix/pickup[31381]: F3B9C98025E: uid=1000 from=
Mar  7 09:39:18 linux1 postfix/smtp[31558]: F3B9C98025E: to=

The key observation here is that Postfix is rewriting root to root@mydomain.com and sending it externally. This occurs because of your myorigin setting combined with the relayhost configuration.

First, let's modify your /etc/postfix/main.cf:

# Add local domain to mydestination
mydestination = linux1.mydomain.com, localhost.linux1.mydomain.com, localhost, $myhostname

# Ensure proper local delivery
local_recipient_maps = proxy:unix:passwd.byname $alias_maps

Your /etc/aliases is actually correct:

root: anothermail@anotherdomain.com
postmaster:    root

But you need to ensure Postfix processes these aliases by:

sudo newaliases
sudo systemctl restart postfix

Verify with these commands:

# Test local delivery
echo "Test local mail" | mail -s "Local Test" root

# Check mail queue
postqueue -p

# Verify aliases
postmap -q root hash:/etc/aliases

If aliases don't work, create /root/.forward:

anothermail@anotherdomain.com

Then set permissions:

chmod 644 /root/.forward
chown root:root /root/.forward

1. Verify mydestination includes local domains
2. Confirm alias_maps points to /etc/aliases
3. Check local_recipient_maps includes aliases
4. Ensure proper file permissions