I agree with your suggestion, so I've already created a quick rule within the conf.d/roundcubemail.conf to force a redirect.
But as a precaution, shouldn't the "%t" parameter at least stop from returning just "tld" ? it would be nice if it stopped at just "domain.tld". Or maybe I'm wrong and some setups require a special kind of login like "user@tld".
Anyway, thank you for your help again, I appreciate it.