/* POSTCONF -N*/
append_dot_mydomain = no
biff = no
broken_sasl_auth_clients = yes
config_directory = /etc/postfix
delay_warning_time = 4h
disable_vrfy_command = yes
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = ipv4
local_recipient_maps = $virtual_mailbox_maps
local_transport = virtual
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
maximal_backoff_time = 8000s
maximal_queue_lifetime = 7d
minimal_backoff_time = 1000s
mydestination = #$mydomain, $myhostname, localhost
myhostname = mail.my_domain.com
mynetworks = 127.0.0.0/8
myorigin = my_domain.com
readme_directory = no
recipient_delimiter = +
relayhost =
smtp_helo_timeout = 60s
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_client_restrictions = reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nl, reject_rbl_client dnsbl.njabl.org
smtpd_delay_reject = yes
smtpd_hard_error_limit = 12
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks, warn_if_reject reject_non_fqdn_hostname, reject_invalid_hostname, permit
smtpd_recipient_limit = 16
smtpd_recipient_restrictions = permit_sasl_authenticated, reject_unauth_pipelining, permit_mynetworks, reject_non_fqdn_recipient, reject_unknown_recipient_domain, reject_unauth_destination, permit smtpd_data_restrictions = reject_unauth_pipelining
smtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain =
smtpd_sasl_security_options = noanonymous
smtpd_sender_restrictions = permit_sasl_authenticated, permit_mynetworks, warn_if_reject reject_non_fqdn_sender, reject_unknown_sender_domain, reject_unauth_pipelining, permit
smtpd_soft_error_limit = 3
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes
unknown_local_recipient_reject_code = 450
virtual_alias_maps = proxy:mysql:/etc/postfix/mysql/virtual_alias_maps.cf
virtual_gid_maps = static:8
virtual_mailbox_base = /var/mail/vmail
virtual_mailbox_domains = proxy:mysql:/etc/postfix/mysql/virtual_domains_maps.cf
virtual_mailbox_limit = 512000000
virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql/virtual_mailbox_maps.cf
virtual_minimum_uid = 8
virtual_transport = virtual
virtual_uid_maps = static:8

/* /etc/postfix/sasl/smtpd.conf*/
pwcheck_method: saslauthd
saslauthd_path: /var/run/saslauthd/mux
log_level: 7
mech_list: PLAIN LOGIN
auxprop_plugin: mysql
allow_plaintext: true
sql_engine: mysql
sql_hostnames: 127.0.0.1
sql_user: postfix
sql_passwd: password
sql_database: postfixadmin
sql_select: select password from mailbox where username = '%u'

Here the error message from syslog : "SASL PLAIN authentication failed: authentication failure"

Here the error messages from auth.log :
May 17 11:51:09 host postfix/smtpd[17405]: sql auxprop plugin using mysql engine
May 17 11:51:09 host saslauthd[17276]: DEBUG: auth_pam: pam_authenticate failed: Authentication failure
May 17 11:51:09 host saslauthd[17276]: do_auth         : auth failure: [user=user@domain.com] [service=smtp] [realm=domain.com] [mech=pam] [reason=PAM auth error]