How to force SMTP AUTH, when Authentication is optional on a SMTP server?

[Mikhail]

Hello,

From the config file:

// SMTP username (if required) if you use %u as the username Roundcube
// will use the current username for login
$config['smtp_user'] = '';

As a part of the requirements, we need to force Roundcube to authenticate towards SMTP server, even though the server doest require it.
Would be grateful if somebody could clarify if it is feasible either possible to configure this behaviour, or to develop en extension/plugin (not to modify the core!) to enable it.

Regards.