Hehe! Ok, I found the reason for all this mess, and the "use your ip instead of hostname" helped me out:
The /etc/hosts file configured inside the chroot is merely used for zimbra to be able to install inside the chroot; but for "external" (out of chroot) requests, the chrooted /etc/hosts file is not considered, but rather the gentoo /etc/hosts file:
This file is different in syntax, containing:
So first line (127.0.0.1) contains 4 entries, 2 of which are also contained in the second line (local IP); I guess that reading the first line also leads to automatically resolving it without looking for further instructions (next lines)... so not finding 192.168.0.1, but rather 127.0.0.1.
127.0.0.1 hostname.domain.foo hostname localhost.localdomain localhost
192.168.0.1 hostname.domain.foo hostname
All I had to do is change my gentoo /etc/hosts to reflect the contents of my chrooted debian jail.
A trivia: I understand not beeing able to connect from outside via public IP - for obvious security reasons - , but why is it that I cannot connect to localhost (127.0.0.1)?