Hello @tahsin karateke,
Unfortunately if the accepting server is bouncing your email because it thinks your IP is on a blacklist (sorbs in this case), there really is not much you can do.
It sounds like you are taking all the right steps to try to remove it, and really contacting sorbs is the ultimate goal. If this is a continuing problem with the same recipients, you can have them ask their IT admins to whitelist either your domain or IP to ensure your mail is delivered.
This type of situation is a very frustrating one to be in, as you are clearly doing everything you can, running DMARC and DKIM and following the right standards, but an external party not accepting your mail because of a ip showing up on a blacklist is something that is entirely outside of your control.
Have you been in possession of the IP for a long time? Is it dedicated?
Regardless, continue to do what you are doing, perhaps contact sorbs and see if they can give you any more details.
Goodluck, i wish there was a more definitive way to pursue this, but there is not.