Cannot send email from Roundcube: 4.7.1 Relay access denied

Symptoms

• It is not possible to send mail via Roundcube:

  PLESK_ERROR: SMTP Error (535): Authentication failed.

• The following error can be found in /var/log/maillog:

  CONFIG_TEXT: postfix/smtpd[30391]: NOQUEUE: reject: RCPT from example.hostname.com[::1]: 454 4.7.1 <john.doe@example.com>: Relay access denied; from=<jane.doe@test.com> to=<john.doe@example.com> proto=ESMTP helo=<webmail.test.com>

Cause

Misconfiguration in Roundcube settings.

There is no user value in Roundcube's configuration file:

# grep smtp_user /usr/share/psa-roundcube/config/main.inc.php
$rcmail_config['smtp_user'] = '';

OR

# grep smtp_user /usr/share/psa-roundcube/config/defaults.inc.php
$config['smtp_user'] = '';

Resolution

1. Connect to the server via SSH

2. Edit config.inc.php file with any text editor and add smtp_user parameter with %u value as follows:

   # grep smtp_user /usr/share/psa-roundcube/config/config.inc.php
   $config['smtp_user'] = '%u';