Troubleshooting a Postfix Installation

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Troubleshooting a Postfix Installation

Understanding a service well enough to troubleshoot it is an important skill as a System Administrator. In this lab, we'll go over troubleshooting a Postfix installation.

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Lab Info

Level

Intermediate

Last updated

Apr 06, 2025

Duration

15m

Challenge

Fix the relay error given during testing

First, we verify the reported issue by conducting our own test.

telnet localhost smtp
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 server1.example.com ESMTP Postfix

ehlo localhost
250-server1.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

mail from: [email protected]
250 2.1.0 Ok

rcpt to: [email protected]
454 4.7.1 <[email protected]>: Relay access denied

quit
221 2.0.0 Bye

Relay access denied means there's something wrong with the postfix configuration. If we look at /etc/postfix/main.cf, we find that the mydestination setting has this:

mydestination = $myhostname, localhost.$mydomain, localhost

It's missing the $mydomain setting at the end of the line. So changing it to:

mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

Then restart postfix to resolve the error.

telnet localhost smtp

Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 server1.example.com ESMTP Postfix

ehlo localhost
250-server1.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

mail from: [email protected]
250 2.1.0 Ok

rcpt to: [email protected]
250 2.1.5 Ok

data
354 End data with <CR><LF>.<CR><LF>

test.
.
250 2.0.0 Ok: queued as 9B26C97CBF

quit
221 2.0.0 Bye
Connection closed by foreign host.

Challenge

Set cloud_user up to receive all of root's email
The easiest way in my experience is to first verify the alias_maps setting in postfix's main.cf.
```
grep alias_maps /etc/postfix/main.cf
...
alias_maps = hash:/etc/aliases
...
```
After making sure that's set to hash:/etc/aliases, we can edit /etc/aliases and make sure there's a root: cloud_user line at the end, and then run the newaliases command.

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