Thank you for your effort! See the output of postconf -n below.
Also note, the example.com domain is NOT the problematic one. example.com is alias for the problematic domain.
(of course example.com is replacement for my real domain name)
alias_database = hash:/etc/postfix/aliases
alias_maps = hash:/etc/postfix/aliases
allow_min_user = no
allow_percent_hack = no
append_dot_mydomain = no
biff = no
bounce_queue_lifetime = 4h
broken_sasl_auth_clients = yes
config_directory = /etc/postfix
content_filter = smtp-amavis:[127.0.0.1]:10024
delay_warning_time = 0h
disable_vrfy_command = yes
dovecot_destination_recipient_limit = 1
enable_original_recipient = no
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = ipv4
mailbox_command = /usr/lib/dovecot/deliver
mailbox_size_limit = 0
maximal_backoff_time = 4000s
maximal_queue_lifetime = 4h
message_size_limit = 15728640
minimal_backoff_time = 300s
mydestination = $myhostname, localhost, localhost.localdomain, localhost.$myhostname
mydomain = example.com
myhostname = email2.example.com
mynetworks = 127.0.0.0/8
mynetworks_style = subnet
myorigin = email2.example.com
proxy_read_maps = $canonical_maps $lmtp_generic_maps $local_recipient_maps $mydestination $mynetworks $recipient_bcc_maps $recipient_canonical_maps $relay_domains $relay_recipient_maps $relocated_maps $sender_bcc_maps $sender_canonical_maps $smtp_generic_maps $smtpd_sender_login_maps $transport_maps $virtual_alias_domains $virtual_alias_maps $virtual_mailbox_domains $virtual_mailbox_maps $smtpd_sender_restrictions
queue_run_delay = 300s
readme_directory = no
recipient_bcc_maps = proxy:pgsql:/etc/postfix/pgsql/recipient_bcc_maps_user.cf, proxy:pgsql:/etc/postfix/pgsql/recipient_bcc_maps_domain.cf
recipient_delimiter = +
relay_domains = $mydestination, proxy:pgsql:/etc/postfix/pgsql/relay_domains.cf
relayhost =
sender_bcc_maps = proxy:pgsql:/etc/postfix/pgsql/sender_bcc_maps_user.cf, proxy:pgsql:/etc/postfix/pgsql/sender_bcc_maps_domain.cf
smtp-amavis_destination_recipient_limit = 1
smtp_data_init_timeout = 240s
smtp_data_xfer_timeout = 600s
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
smtpd_data_restrictions = reject_unauth_pipelining
smtpd_end_of_data_restrictions = check_policy_service inet:127.0.0.1:10031
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_non_fqdn_helo_hostname, reject_invalid_helo_hostname, check_helo_access pcre:/etc/postfix/helo_access.pcre
smtpd_recipient_restrictions = reject_unknown_sender_domain, reject_unknown_recipient_domain, permit_sasl_authenticated, reject_non_fqdn_sender, reject_non_fqdn_recipient, reject_unlisted_recipient, check_policy_service inet:127.0.0.1:7777, check_policy_service inet:127.0.0.1:10031, permit_mynetworks, reject_unauth_destination
smtpd_reject_unlisted_recipient = yes
smtpd_reject_unlisted_sender = yes
smtpd_sasl_auth_enable = yes
smtpd_sasl_authenticated_header = no
smtpd_sasl_local_domain =
smtpd_sasl_path = ./dovecot-auth
smtpd_sasl_security_options = noanonymous
smtpd_sasl_type = dovecot
smtpd_sender_login_maps = proxy:pgsql:/etc/postfix/pgsql/sender_login_maps.cf
smtpd_sender_restrictions = permit_mynetworks, reject_authenticated_sender_login_mismatch, permit_sasl_authenticated
smtpd_tls_CAfile = /etc/ssl/certs/iRedMail_CA.pem
smtpd_tls_always_issue_session_ids = no
smtpd_tls_cert_file = /etc/ssl/certs/example.com-combined.crt
smtpd_tls_key_file = /etc/ssl/private/example.com.key
smtpd_tls_loglevel = 0
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes
swap_bangpath = no
tls_random_source = dev:/dev/urandom
transport_maps = proxy:pgsql:/etc/postfix/pgsql/transport_maps_user.cf, proxy:pgsql:/etc/postfix/pgsql/transport_maps_domain.cf
virtual_alias_domains =
virtual_alias_maps = proxy:pgsql:/etc/postfix/pgsql/virtual_alias_maps.cf, proxy:pgsql:/etc/postfix/pgsql/domain_alias_maps.cf, proxy:pgsql:/etc/postfix/pgsql/catchall_maps.cf, proxy:pgsql:/etc/postfix/pgsql/domain_alias_catchall_maps.cf
virtual_gid_maps = static:2000
virtual_mailbox_base = /home/vmail
virtual_mailbox_domains = proxy:pgsql:/etc/postfix/pgsql/virtual_mailbox_domains.cf
virtual_mailbox_maps = proxy:pgsql:/etc/postfix/pgsql/virtual_mailbox_maps.cf
virtual_minimum_uid = 2000
virtual_transport = dovecot
virtual_uid_maps = static:2000