I know nothing of your organisation, and all I know about SORBS is what I have gleaned from their website. I will not comment on the rights or wrongs of the situation, just the technical aspects
I would hazard from the little information that you have provided that you have your own email server.
It appears that email communication between your server and the destination email server in the parent company is being blocked by an intermediate mail relay server because that intermediate subscribes to SORBS.
There will be at least 4 mail servers in the chain - yours, your ISP, the German ISP and the Parent email server. There may well be more.
It is unlikely to be the Parent email server that is blocking you, and I'm sure that your ISP would have alerted you if they were blocking you, so the probability is that the German ISP is the SORBS subscriber - or else an additional intermediate relay server.
It would appear unlikely that it is an arbitrary block, it could be because your email server (or your ISP's mail relay) is incorrectly configured and is vulnerable to mail relay abuse, and as such the SORBS-enabled intermediary has blocked your email. I would have to say that the balance of probability is that it is your email server.
Assuming that you have control over your own email server, then you should check and re-check your server configuration (or hire someone to do so). Then request a re-test from SORBS. If you don't host your own email server, then it is up to your ISP to sort it out.
An alternative strategy might be to set up a VPN between your office and the Parent office. That would give you a number of options - for example, you could directly connect an email server to the Parent system, keeping your existing email server to connect to the rest of the world, or dispense with the existing email server and route all your email via the Parent system etc. etc.
SD