Thanks for the feedback. I checked the package (available in openSUSE:Backports:SLE-15-SP5:Update/roundcubemail aka https://build.opensuse.org/package/show/openSUSE:Maintenance:18166/roundcubemail.openSUSE_Backports_SLE-15-SP5_Update ) and also the released binary RPM (available in http://download.opensuse.org/update/leap/15.5/backports/noarch/roundcubemail-1.6.4-bp155.2.6.1.noarch.rpm ). What wonders me: we ship the official config.php(.sample), that is also provided by Roundcube upstream. There is no 'smtp_port' config option in the whole config sub-directory. My suspicion would be an upgraded Roundcubemail package. But you are talking about a new installation via Yast, so I have currently not a single idea how this can happen. Can you please verify that this is really a fresh installation and not an update/upgrade?