Download the latest Cyrus SASL2 package.

On Debian/Ubuntu systems you can do

% sudo apt-get install sasl2-bin libsasl2-modules

Edit: /etc/postfix/sasl/smtpd.conf

mech_list: PLAIN LOGIN ANONYMOUS
pwcheck_method: saslauthd

Edit: /etc/default/saslauthd and check the following lines

START=yes
MECHANISMS="rimap"
MECH_OPTIONS="localhost"

saslauthd will make an additional IMAP connection to local IMAP server every time postfix attempts to authenticate a user.

On Redhat-like systems the saslauthd config is done via /etc/sysconfig/saslauthd :

SOCKETDIR=/var/run/saslauthd
MECH=rimap
FLAGS="-O 127.0.0.1"

Follow the doc/README.postfix in the source distribution for DBMail and first make sure delivery works as required.

Activate SMTP/AUTH by setting some parameters. One way to do that is by editing main.cf which for Debian/Ubuntu resides in /etc/postfix/main.cf

smtpd_sasl_auth_enable = yes

this will activate smtp-auth

smtpd_recipient_restrictions = 
        permit_mynetworks
        reject_invalid_hostname
        reject_non_fqdn_sender
        reject_non_fqdn_recipient
        reject_unknown_recipient_domain
        reject_unknown_sender_domain
        reject_unauth_pipelining
        permit_sasl_authenticated
        reject_unauth_destination

Here the actual settings for this parameter may be different in your case. But make sure the 'permit_sasl_authenticated' option is in there, before the 'reject_unauth_destination' option.

Restart saslauthd and postfix, and SMTP-Auth should work!

/etc/init.d/saslauthd restart
/etc/init.d/postfix reload