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Labs

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 platform
Lab Info
Level
Intermediate
Last updated
Apr 06, 2025
Duration
15m

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Table of Contents
  1. 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.
    
  2. 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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