ok, well, then there seems to be something wrong...I put the FQDN as smtp and IMAP, both tried with and without SSL, but it doesn't work.
The Domain has it's own IP (from 3) and in DNS-Manager the Nameservers are (is that correct?) both the same (FQDN from first IP I used and installed ISPConfig on). In the Records the IP for the Domain, I'm testing, is the one I wanted this Domain to be onand the only other entrance here is the Mailserver which again points with it's FQDN to the main ISPConfig-IP/Domain.
I can use RoundCube without errors, but to get in from the outside, there seems something misconfigured by me?
The provider gave me for the 3 IP's 3 own Reserve-DNS entrances....maybe I would have to use the one for the particular Domain/IP in the Records?