i am using XenCenter, that's the application which is the standard management program for XenServer (runs on Windows only) - it includes a console which acts like an SSH session, at least it does look like it.
Makes sense that this is related to the problem, as the error which puts itself into the mail box name is about screen resolution problems.
On CentOS, the installer looks different on that console (no colors), even though it also misbehaves.
As mentioned, I did manage to install iRedMail on Ubuntu anyway in the end - just clicking Enter on that screen.
I'll try it on a physical server still if I get around to it, but otherwise thanks for the assistance.