Postfix random delivery problem

Hi everyone, lately I noticed that sometimes Postfix seems to ignore the pr=sence of a .forward file in the user folder, and so hands out the message =o the lmtpunix delivery program, which doesn't find a cyrus mailbox folder=and replies to the sender that the user is not known with 550-Mailbox unkn=wn.

Does everybody have any suggestion?

Thanks in advance
R.

Following I attached the error and the main.cf


Apr 12 11:27:30 mail postfix/lmtp[7321]: AF0F41EDBD2: to=<fpa@MYDOMAIN.com=>, orig_to=<name.last@DOMAIN.com>, relay=mail.MYDOMAIN.com[/var/lib/im=p/socket/lmtp], delay=0.2, delays=0.12/0/0.02/0.05, dsn=5.1.1, statu==bounced (host mail.MYDOMAIN.com[/var/lib/imap/socket/lmtp] said: 550-Ma=lbox unknown. Either there is no mailbox associated with this 550-name or=you do not have authorization to see it. 550 5.1.1 User unknown (in reply =o RCPT TO command))


alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
broken_sasl_auth_clients = yes
command_directory = /usr/sbin
config_directory = /etc/postfix
content_filter = filter:127.0.0.1:10025
daemon_directory = /usr/libexec/postfix
debug_peer_level = 10
disable_vrfy_command = yes
header_checks = regexp:/etc/postfix/header_checks
html_directory = no
inet_interfaces = localhost, thismail
local_destination_concurrency_limit = 10
local_destination_recipient_limit = 300
mail_owner = postfix
mailbox_transport = lmtp:unix:/var/lib/imap/socket/lmtp
mailq_path = /usr/bin/mailq.postfix
manpage_directory = /usr/share/man
message_size_limit = 40960000
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain, M=DOMAIN.com, mail.$mydomain, www.$mydomain, f
tp.$mydomain, sirio.$mydomain, mail-router.$mydomain
mydomain = MYDOMAIN.com
myhostname = mail.MYDOMAIN.com
mynetworks = 168.100.10.0/24, 127.0.0.0/8, 192.168.200.0/24
myorigin = $mydomain
newaliases_path = /usr/bin/newaliases.postfix
parent_domain_matches_subdomains = yes
queue_directory = /var/spool/postfix
readme_directory = /usr/share/doc/postfix-2.3.3/README_FILES
receive_override_options = no_address_mappings
relay_domains = $mydestination
sample_directory = /usr/share/doc/postfix-2.3.3/samples
sendmail_path = /usr/sbin/sendmail.postfix
setgid_group = postdrop
smtpd_delay_reject = no
smtpd_helo_required = yes
smtpd_recipient_restrictions = check_client_access hash:/etc/postfix/acce=s, check_helo_access hash:/etc/postfix/helo_access,
permit_mynetworks, permit_sasl_authenticated, reject_unaut=_pipelining, reject_unauth_destination, r
eject_non_fqdn_hostname, reject_non_fqdn_recipient, reject_inva=id_hostname, reject_non_fqdn_sender, reject_r
bl_client sbl-xbl.spamhaus.org, check_policy_service inet:127.0.0.1:10023, = permit
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_type = cyrus
smtpd_tls_auth_only = yes
smtpd_tls_cert_file = /etc/pki/tls/certs/mail.MYDOMAIN.com.cert
smtpd_tls_key_file = /etc/pki/tls/private/mail.MYDOMAIN.com.key
smtpd_tls_loglevel = 1
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:/var/spool/postfix/smtpd_tls_cac=e
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom
transport_maps = hash:/etc/postfix/transport
unknown_hostname_reject_code = 550
unknown_local_recipient_reject_code = 550