Scenario
Your mail server hosts multiple domains.
You use a MySQL database as backend for user authentication.

Users authenticate to the POP3/IMAP server as :
username : username@example.org
password : test123

You want to allow them to authenticate with SMTP AUTH using the same credentials.
The credentials are stored in a MySQL database in an encrypted form.
You want them to authenticate on a secure channel (TLS)

What you need
postfix with MySQL, TLS support (available in the centosplus repository)
cyrus-sasl (available in the base repo)
cyrus-sasl-plain (ditto)
cyrus-sasl-md5 (ditto)
openssl (ditto)
pam_mysql (ditto)

pam_mysql is available at : http://repo.securityteam.us/repository/
el3 : http://repo.securityteam.us/repository/redhat/el3/i386/RPMS/pam_mysql-0.6.2-2.i386.rpm
el4 : http://repo.securityteam.us/repository/redhat/el4/i386/RPMS/pam_mysql-0.6.2-2.i386.rpm

How to set it up
* Edit /etc/pam.d/smtp :

#%PAM-1.0
#auth required pam_stack.so service=system-auth
#account required pam_stack.so service=system-auth
# Comment : SMTP AUTH with MySQL
# This is the first line
auth required pam_mysql.so user=database passwd=password host=127.0.0.1 db=postfix table=mailbox usercolumn=username passwdcolumn=password crypt=1 md5=1
# This is a second line
account sufficient pam_mysql.so user=database passwd=password host=127.0.0.1 db=postfix table=mailbox usercolumn=username passwdcolumn=password crypt=1 md5=1

Change the info to match your configuration
The auth and account lines could be cut in half in this article.
If you are copy-pasting the text, make sure everything holds on one line on both auth and account statement.

* Edit /usr/lib/sasl2/smtpd.conf :

pwcheck_method: saslauthd
# add the following two lines for smtp auth
mech_list: PLAIN LOGIN
log_level: 5

* Make sure “MECH=pam” is present under /etc/sysconfig/saslauthd

===
Edit 15 dec 2006 :
IMPORTANT NOTICE FOR RHEL/CENTOS 4 USERS

I’m currently installing a mail server under CentOS 4.4 while this guide describes the CentOS 3 way
I thought it did not matter but there’s a small difference with saslauthd..

Under CentOS 4.x you need to add the following line in /etc/sysconfig/saslauthd :
FLAGS="-r"

Without the -r flag, saslauthd would query the MySQL database this way :
25 Query SELECT password FROM mailbox WHERE username = 'admin'

Obviously, the whole email address is stored in the database, with the -r flag, it will query correctly :
26 Query SELECT password FROM mailbox WHERE username = 'admin@example.org'

The man page for saslauthd mentions the -r flag under CentOS 4 only..

-r Combine the realm with the login (with an ’@’ sign in between). e.g. login: “foo” realm: “bar” will get passed as login: “foo@bar”. Note that the realm will still be passed, which may lead to unexpected behavior.
===

* Add the following lines to /etc/postfix/main.cf :

smtpd_sasl_local_domain =
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes

This will enable SASL authentication to all the smtp ssl instances.
You can disable sasl auth for certain instances under /etc/postfix/master.rc if you need

10.0.0.1:smtps inet n - n - 10 smtpd
-o smtpd_sasl_auth_enable=no

* Under /etc/postfix/main.cf :

Add “permit_sasl_authenticated” to the “smtpd_recipient_restrictions” section :

smtpd_recipient_restrictions =
...
permit_sasl_authenticated
reject_unauth_destination

Enable TLS

* Finally create the SSL certificate needed by TLS:

# mkdir /etc/postfix/tls
# cd /etc/postfix/tls
# openssl genrsa -des3 -rand /etc/hosts -out smtpd.key 1024
# openssl req -new -key smtpd.key -out smtpd.csr

Note: leave “challenge password” empty.

# openssl x509 -req -days 3650 -in smtpd.csr -signkey smtpd.key -out smtpd.crt

# openssl rsa -in smtpd.key -out smtpd.key.unencrypted

# mv -f smtpd.key.unencrypted smtpd.key

# openssl req -new -x509 -extensions v3_ca -keyout cakey.pem -out cacert.pem -days 3650

* Add the following lines to /etc/postfix/main.cf:

smtpd_tls_auth_only = no
# smtp_use_tls : if set to "no" do not use TLS when connecting to server offering STARTTLS
smtp_use_tls = no
# smtpd_use_tls : if set to "yes", offers STARTTLS to SMTP clients
smtpd_use_tls = yes
# smtp_tls_note_starttls_offer : logs STARTTLS offers from remote SMTP server
smtp_tls_note_starttls_offer = yes
smtpd_tls_key_file = /etc/postfix/tls/smtpd.key
smtpd_tls_cert_file = /etc/postfix/tls/smtpd.crt
smtpd_tls_CAfile = /etc/postfix/tls/cacert.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom
tls_daemon_random_source = dev:/dev/urandom

Restart the services
service saslauthd restart
service postfix restart

Start services at boot
chkconfig postfix on
chkconfig saslauthd on

Debugging issues with verbose logs :

* pam-mysql :
add “verbose=true” to the end of the auth and account lines in /etc/pam.d/smtp

* saslauthd :
stop the service
start saslauthd manually :
# saslauthd -a pam -d