SMTP Error (454): Failed to add recipient Relay access denied roundcube - email

Question

I setup a mail server on VPS and I installed roundcube to manage webmail. when i send e-mail in intern it work's but when i try to send an e-mail to another mail server (gmail) i get this error :
SMTP Error (454): Failed to add recipient "user#gmail.com" (4.7.1 : Relay access denied).
but when i send the same e-mail with thunderbird it works so here what i found at logs:
with roundcube:
Dec 4 18:44:09 vpsxxxxxx postfix/smtpd[3013]: connect from xxx.xxx.xxx.xxx
Dec 4 18:44:09 vpsxxxxxx postfix/smtpd[3013]: warning: connect to Milter service inet:127.0.0.1:8891: Connection refused
Dec 4 18:44:09 vpsxxxxxx postfix/smtpd[3013]: NOQUEUE: reject: RCPT from xxx.xxx.xxx.xxx: 454 4.7.1 <user#gmail.com>: Relay access denied; from=<user#domain.com> to=<user#gmail.com> proto=ESMTP helo=<mail.domain.com>
Dec 4 18:44:09 vpsxxxxxx postfix/smtpd[3013]: disconnect from xxx.xxx.xxx.xxx ehlo=1 mail=1 rcpt=0/1 rset=1 quit=1 commands=4/5
with thunderbird :
Dec 4 18:44:55 vpsxxxxxx postfix/smtpd[3016]: connect from unknown[xxx.xxx.xxx.xxx]
Dec 4 18:44:55 vpsxxxxxx postfix/smtpd[3016]: warning: connect to Milter service inet:127.0.0.1:8891: Connection refused
Dec 4 18:44:58 vpsxxxxxx postfix/smtpd[3016]: 64DCD5EEC4: client=unknown[xxx.xxx.xxx.xxx], sasl_method=PLAIN, sasl_username=user#domain.com
Dec 4 18:44:58 vpsxxxxxx postfix/cleanup[3019]: 64DCD5EEC4: message-id=<9bf8a7ca-9d6a-847c-5350-0f5c4350951f#domain.com>
Dec 4 18:44:58 vpsxxxxxx postfix/qmgr[2865]: 64DCD5EEC4: from=<user#domain.com>, size=1810, nrcpt=1 (queue active)
Dec 4 18:44:58 vpsxxxxxx postfix/smtpd[3016]: disconnect from unknown[xxx.xxx.xxx.xxx] ehlo=2 starttls=1 auth=1 mail=1 rcpt=1 data=1 quit=1 commands=8
Dec 4 18:44:59 vpsxxxxxx postfix/smtp[3020]: connect to gmail-smtp-in.l.google.com[2a00:1450:400c:c07::1b]:25: Cannot assign requested address
Dec 4 18:45:00 vpsxxxxxx postfix/smtp[3020]: 64DCD5EEC4: to=<user#gmail.com>, relay=gmail-smtp-in.l.google.com[66.102.1.27]:25, delay=1.7, delays=0.25/0.01/0.52/0.91, dsn=2.0.0, status=sent (250 2.0.0 OK 1543945500 f16si12703624wrt.428 - gsmtp)
Dec 4 18:45:00 vpsxxxxxx postfix/qmgr[2865]: 64DCD5EEC4: removed
Dec 4 18:45:01 vpsxxxxxx dovecot: imap-login: Login: user=<user#domain.com>, method=PLAIN, rip=xxx.xxx.xxx.xxx, lip=yyy.yyy.yyy.yyy, mpid=3025, TLS, session=<HDgM0zV8EcXE5dog>
Dec 4 18:45:01 vpsxxxxxx dovecot: imap-login: Login: user=<user#domain.com>, method=PLAIN, rip=xxx.xxx.xxx.xxx, lip=yyy.yyy.yyy.yyy, mpid=3026, TLS, session=<dIQM0zV8EsXE5dog>
Dec 4 18:45:01 vpsxxxxxx dovecot: imap(user#domain.com): Logged out in=1669 out=552
config.inc.php
<?php
$config['db_dsnw'] = 'mysql://databaseuser:password#localhost/database';
$config['default_host'] = 'localhost';
$config['smtp_server'] = 'mail.domain.com';
$config['support_url'] = '';
$config['ip_check'] = true;
$config['des_key'] = 'DES_KEY';
$config['username_domain'] = 'domain.com';
$config['product_name'] = 'domain Webmail';
$config['plugins'] = array(
'acl', 'additional_message_headers', 'archive', 'attachment_reminder', 'autologon',
'debug_logger', 'emoticons', 'enigma', 'example_addressbook', 'help', 'hide_blockquote',
'http_authentication', 'identicon', 'identity_select', 'jqueryui', 'krb_authentication',
'managesieve', 'markasjunk', 'new_user_dialog', 'new_user_identity', 'newmail_notifier',
'password', 'redundant_attachments', 'show_additional_headers', 'squirrelmail_usercopy',
'subscriptions_option', 'userinfo', 'vcard_attachments', 'virtuser_file', 'virtuser_query', 'zipdownload'
);
$config['language'] = 'en_EN';
$config['enable_spellcheck'] = false;
?>
domain.com is my Domain name

Answer 1

It was a configuration problem. I add this to config.inc.php, and it works
$config['smtp_server'] = 'tls://mail.domain.com';
$config['smtp_port'] = 587;
$config['smtp_user'] = '%u';
$config['smtp_pass'] = '%p';
$config['smtp_conn_options'] = array(
'ssl' => array(
'verify_peer' => false,
'verify_peer_name' => false,
),
);