IMAP authentication / SASL errors (warning: localhost.localdomain[127.0.0.1]: SASL LOGIN authentication failed: authentication failure)

Plesk 11.x have fix for this but you can fix this error on 10.x also as below :)

mv /usr/lib64/sasl2/smtpd.conf /usr/lib64/sasl2/smtpd.conf.$(date +%s)
cat <<'SASL_CONF_EOF'>/usr/lib64/sasl2/smtpd.conf
pwcheck_method: auxprop saslauthd
auxprop_plugin: sql_sqlite3
saslauthd_path: /var/spool/postfix/private/plesk_saslauthd
mech_list: DIGEST-MD5 CRAM-MD5 PLAIN LOGIN
auto_transition: yes
sql_engine: sqlite3
sql_hostnames: localhost
sql_database: /var/spool/postfix/plesk/passwd.db
sql_select: SELECT `%p` FROM domains d, users u WHERE u.name='%u' and d.name='%r' and d.status=0 and u.status=0 and u.dom_id=d.id
sql_verbose: yes
log_level: 9
SASL_CONF_EOF
service saslauthd restart
service postfix restart