I have the exact same problem, using servage.net. I tried with "user@domain.tdl" instead of %u, and I have tried using all the different AUTH types, to no avail. Any suggestions?
// use this host for sending mails.
// to use SSL connection, set ssl://smtp.host.com
// if left blank, the PHP mail() function is used
$rcmail_config['smtp_server'] = 'smtp1.servage.net';
// SMTP port (default is 25; 465 for SSL)
$rcmail_config['smtp_port'] = 5190;
// SMTP username (if required) if you use %u as the username RoundCube
// will use the current username for login
$rcmail_config['smtp_user'] = '%u';
// SMTP password (if required) if you use %p as the password RoundCube
// will use the current user's password for login
$rcmail_config['smtp_pass'] = '%p';
// SMTP AUTH type (DIGEST-MD5, CRAM-MD5, LOGIN, PLAIN or empty to use
// best server supported one)
$rcmail_config['smtp_auth_type'] = '';