I successfully switched my hosting from my real-hardware machine ‘daemon’ to my new virtual-hardware machine ‘gauss’ which is hosted by HostEurope.de. After turning off the services on ‘daemon’ and doing a final sync, I started the services on ‘gauss’ and activated the TCP-forwarding on ‘daemon’ with rinetd. I forwarded FTP, HTTP(S) and IMAP(S) such that users could connect to the old IP where those connections got forwarded to the new IP. With this, the new host only saw connections originating from the old IP although they actually belonged to strangers. Therefore I didn’t also forward the SMTP-port directly, because in that case the realtime blacklisting would have failed.
So I configured Postfix to keep doing RBL lookups and SPAM and virus scanning, and to forward mails destined for the local domains to the new host. I did it this way: