Command Center

Enabling SMTP authentication in Postfix ensures that only users with valid accounts can send email outside your network (relaying). This prevents spammers from using your SMTP server as a spam broadcast station. Here’s how to do it in Postfix.

If you are using Red Hat Enterprise Linux 5 or CentOS 5, please read Postfix SMTP Authentication and Dovecot SASL instead. It’s a lot easier to setup and you won’t have to duplicate your Dovecot authentication setup into SASL.

Configure SASL

1. Edit the file /usr/lib/sasl2/smtpd.conf (/usr/lib64/sasl2/smtpd.conf for 64-bit users) and add the line below to the bottom of the file

mech_list: PLAIN LOGIN

2. Start the saslauthd service.

Configure Postfix

1. Click Applications then click File Browser. This will launch the File Browser window.

2. In the Location field, type in /etc/postfix and press Enter.

3. Double click on the file main.cf to open it for editing.

mynetworks = 127.0.0.0/8
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions =  permit_mynetworks,
  permit_sasl_authenticated, reject_unauth_destination
broken_sasl_auth_clients = yes

4. Restart the Postfix service or the MailScanner service if you have integrated MailScanner into Postfix.

Test Postfix

In a Terminal window, type in the highlighted commands below.

Sample postfix session

[root@mail ~]# telnet mail smtp

Replace mail with the name of your server. We should not use localhost since localhost is a trusted client ip address.

Trying 192.168.0.4...
Connected to mail.acme.local (192.168.0.4).
Escape character is '^]'.
220 mail.acme.local ESMTP Postfix
ehlo host
250-mail.acme.local
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-AUTH LOGIN PLAIN
250-AUTH=LOGIN PLAIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN

Note the new 250-AUTH lines.

mail from: johndoe
250 2.1.0 Ok
rcpt to: test@domain.local
554 5.7.1 : Relay access denied

It works, now to check if we can send it after authenticating.

auth plain AGpvaG5kb2UAcGFzc3dvcmQ=
235 2.0.0 Authentication successful
rcpt to: test@domain.local
250 2.1.5 Ok
quit
221 2.0.0 Bye
Connection closed by foreign host.
[root@mail ~]#

Text highlighted in green only appears in Postfix version 2.3 or higher. Postfix version 2.3 is included in Red Hat Enterprise Linux 5 or CentOS 5.

You can send to email addresses belonging to your domain without authentication. This is normal as it enables you to receive mail from the outside.

The gibberish text after AUTH PLAIN is the base64 encoded value of the user name johndoe and password password. You can generate your own base64 text using the form below.

REFERENCE

http://www.linuxmail.info/smtp-authentication-postfix-centos-5/