postfix 550 relay not permitted for virtual mailbox domain

When somebody sends mail to myvirtualdomain.com I get the message

Remote host said: 550 relay not permitted! [RCPT_TO]

What is wrong with my settings?

I do not think the problem is in the mysql cf-files nor in the database as these settings used to work correctly and have never been changed. Unfortunately I cannot reproduce config changes else where. But if necessay I am happy to provide the content of these files/database.

My server domain (mydomain.com) does receive mail correctly and my main.cf reads as below:

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = /usr/share/doc/postfix

# TLS parameters
smtpd_tls_cert_file = /etc/postfix/smtpd.cert
smtpd_tls_key_file = /etc/postfix/smtpd.key
smtpd_use_tls = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = mail.mydomain.com
mydomain = mydomain.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = 
relayhost = 
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
html_directory = /usr/share/doc/postfix/html
message_size_limit = 30720000
virtual_alias_domains = 
virtual_alias_maps = proxy:mysql:/etc/postfix/mysql-virtual_forwardings.cf, mysql:/etc/postfix/mysql-virtual_email2email.cf
virtual_mailbox_domains = proxy:mysql:/etc/postfix/mysql-virtual_domains.cf
virtual_mailbox_maps = proxy:mysql:/etc/postfix/mysql-virtual_mailboxes.cf
virtual_mailbox_base = /home/vmail
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_sasl_authenticated_header = yes
# see under Spam smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
proxy_read_maps = $local_recipient_maps $mydestination $virtual_alias_maps $virtual_alias_domains $virtual_mailbox_maps $virtual_mailbox_domains $relay_recipient_maps $relay_domains $canonical_maps $sender_canonical_maps $recipient_canonical_maps $relocated_maps $transport_maps $mynetworks $virtual_mailbox_limit_maps
virtual_transport = dovecot
dovecot_destination_recipient_limit = 1

# Spam
disable_vrfy_command = yes
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks,
   check_helo_access hash:/etc/postfix/helo_access,
   reject_non_fqdn_hostname,
   reject_invalid_hostname,
   permit
smtpd_recipient_restrictions =
   permit_mynetworks,
   permit_sasl_authenticated,
   reject_unauth_destination,
   reject_invalid_hostname,
   reject_non_fqdn_sender,
   reject_non_fqdn_recipient,
   reject_unknown_sender_domain,
   reject_unknown_recipient_domain,
   reject_non_fqdn_hostname,
   reject_rbl_client sbl.spamhaus.org,
   reject_rbl_client zen.spamhaus.org,
   reject_rbl_client cbl.abuseat.org,
   reject_rbl_client bl.spamcop.net,
   permit
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20