Postfix 451 4.3.5 Server configuration error

So I'm running mail on my VPS, using Postfix + Dovecot. I am able to send, but not all mail comes in.

Note: DOMAIN = my domain, but I blocked it for security reasons.

The case: I want to use my mail on Twitter, but the verification email never arrived. I checked the log and I found the following:

Aug 20 22:20:51 DOMAIN postfix/smtpd[32679]: warning: unknown smtpd restriction: "ACCEPT"
Aug 20 22:20:51 DOMAIN postfix/smtpd[32679]: NOQUEUE: reject: RCPT from spring-chicken-ae.twitter.com[199.16.156.144]: 451 4.3.5 Server configuration error; from=<b0398d9890aadmin=DOMAIN.nl@bounce.twitter.com> to=<admin@DOMAIN.nl> proto=ESMTP helo=<spring-chicken-ae.twitter.com>
Aug 20 22:20:56 DOMAIN postfix/cleanup[32684]: DE1291BFC5E: message-id=<20170820202056.DE1291BFC5E@DOMAIN.nl>
Aug 20 22:20:56 DOMAIN postfix/qmgr[27377]: DE1291BFC5E: from=<double-bounce@DOMAIN.nl>, size=1344, nrcpt=1 (queue active)
Aug 20 22:20:56 DOMAIN postfix/smtpd[32679]: disconnect from spring-chicken-ae.twitter.com[199.16.156.144] ehlo=2 starttls=1 mail=1 rcpt=0/1 quit=1 commands=5/6
Aug 20 22:20:56 DOMAIN postfix/virtual[32685]: DE1291BFC5E: to=<postmaster@DOMAIN.nl>, orig_to=<postmaster>, relay=virtual, delay=0.02, delays=0.01/0/0/0, dsn=2.0.0, status=sent (delivered to maildir)
Aug 20 22:20:56 DOMAIN postfix/qmgr[27377]: DE1291BFC5E: removed

My main.cf is as follows:

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

home_mailbox = Maildir/
virtual_mailbox_domains = /etc/postfix/vhosts
virtual_mailbox_base = /home/vmail
virtual_mailbox_maps = hash:/etc/postfix/vmaps
virtual_minimum_uid = 1000
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000

milter_protocol = 2
milter_default_action = accept
smtpd_milters = inet:localhost:12301
non_smtpd_milters = inet:localhost:12301

smtp_tls_security_level = encrypt

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

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 = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.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
#smtpd_sasl_security_options = noanonymous
#smtpd_sasl_local_domain = $myhostname
#smtpd_sasl_application_name = smtpd
#broken_sasl_auth_clients = yes
#smtpd_tls_received_header = yes

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = DOMAIN.nl
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname, mail.DOMAIN.nl, localhost.nl, , localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
local_transport = virtual
local_recipient_maps = $virtual_mailbox_maps
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination,check_recipient_access hash:/etc/postfix/filtered_domains
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth-client

The question: What causes this error?