Postfix Sendmail – Troubleshooting Mail Bounces in Ubuntu

The emails are not being sent to email addresses with domainname.co as domain name.

I have in Ubuntu system three users namely root, ubuntu & domainname.

The following is the log(tail /var/log/mail.log -n 20)

Feb 19 04:36:56 server postfix/pickup[22963]: 521A0106D3A: uid=1003 from=<domainname>
Feb 19 04:36:56 server postfix/cleanup[26043]: 521A0106D3A: message-id=<20200219043656.521A0106D3A@server.domainname.co>
Feb 19 04:36:56 server postfix/qmgr[14300]: 521A0106D3A: from=<domainname@domainname.co>, size=357, nrcpt=1 (queue active)
Feb 19 04:36:58 server postfix/smtp[26045]: 521A0106D3A: to=<dushyant.joshi@domainname.co>, relay=in.hes.trendmicro.com[54.219.191.20]:25, delay=1.9, delays=0.01/0.01/1.6/0.21, dsn=5.7.1, status=bounced (host in.hedmicro.com[54.219.191.20] said: 550 5.7.1 <weborders@domainname.co>: Recipient address rejected: ERS-DUL. (in reply to RCPT TO command))
Feb 19 04:36:58 server postfix/cleanup[26043]: 732DD106D41: message-id=<20200219043658.732DD106D41@server.domainname.co>
Feb 19 04:36:58 server postfix/bounce[26046]: 521A0106D3A: sender non-delivery notification: 732DD106D41
Feb 19 04:36:58 server postfix/qmgr[14300]: 732DD106D41: from=<>, size=2368, nrcpt=1 (queue active)
Feb 19 04:36:58 server postfix/qmgr[14300]: 521A0106D3A: removed
Feb 19 04:36:59 server postfix/smtp[26045]: 732DD106D41: to=<domainname@domainname.co>, relay=in.hes.trendmicro.com[54.219.191.21]:25, delay=1.5, delays=0/0/1.3/0.21, dsn=5.7.1, status=bounced (host in.hes.trendmicro.com[54.219.191.21] said: 550 5.7.1 <domainname@domainname.co>: Recipient address rejected: ERS-DUL. (in reply to RCPT TO command))
Feb 19 04:37:00 server postfix/qmgr[14300]: 732DD106D41: removed

The content of the /etc/postfix/main.cf

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
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_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = server.domainname.co
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
#myorigin = /etc/mailname
myorigin = $mydomain
mydestination = $myhostname, server, localhost.domainname.co, localhost
#smtp_generic_maps = texthash:/etc/postfix/generic
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
virtual_alias_maps = hash:/etc/postfix/virtual
sender_bcc_maps = hash:/etc/postfix/bcc
mailbox_command = /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
home_mailbox = Maildir/
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
smtp_tls_security_level = may
allow_percent_hack = no

I am checking this with below script.

$sender = 'sales@domainname.co';
$recipient = 'dushyant.joshi@domainname.co';

$subject = "php mail test";
$message = "php test message";
$headers = 'From:' . $sender;

if (mail($recipient, $subject, $message, $headers)) {
        echo "Message accepted";
} else {
        echo "Error: Message not accepted";
}

UPDATE: The current answer will not help as the error code changed after some configuration changes in main.cf file.