Roundcube 220 Auth Failure

Greetings Till and team -

We host ISPC 3.2 on Ubuntu 20.04/Apache2/MariaDB, on an AWS instance. Everything working smoothly, including postfix/dovecot, as evidenced by successful send & receives via a client SMTP app (Thunderbird). Except for roundcube. We can log in fine, but any attempts to send return

Code:

SMTP Error (220): Authentication Failed

Critical settings in /etc/roundcube/config.inc.php:

Code:

$config['default_host'] = 'localhost';
$config['smtp_server'] = 'tls://%h';
$config['smtp_port'] = 587;
$config['smtp_user'] = '%u';
$config['smtp_pass'] = '%p';
$config['support_url'] = '';
$config['smtp_conn_options'] = array(
  'ssl' => array(
    'verify_peer'  => true,
    'verify_depth' => 3,
    'cafile'       => '/etc/ssl/certs/SectigoRSADomainValidationSecureServerCA.crt',
  ),
);

We have tried all the tweaks suggested (and a few not), but utterly stuck on the 220 error. Any suggestions?

Thanks as always ~

 

Nicely explained - that makes perfect sense. (e.g., that's why postfix works, because it's configured under our 'real' hostname, which has an associated tls cert. )

Thanks so much!