If its Hetzner, they have a bad mail reputation as a result of failing to address issues with their shared resources in a timely manner.

They had at one point a persistent downgrade in mail reputation to the point where it was almost impossible to keep a working mail server with them that would be accepted by any major ESP.

They weren't particularly receptive to addressing support issues where their systems were breaking guidelines/RFCs impacting reputation, at least when I spoke with them about one of my servers a year or so back (which I promptly migrated to another provider).

From what I understand, there were egregious issues. Some of the rumors included source address validation not being done allowing DDOS and spoofing originating from these shared servers on the network block, issues with published PTR records, and a few other things. All of which heavily contribute to mail deliver-ability issues.

> They are sending us email, we forward it, Gmail throttles it because it looks like spam and then they don't accept the bounce for example.

If you are acting as a relay and forwarding mail from Google, to another Google recipient, you need to follow the mandatory guidelines.

https://support.google.com/a/answer/81126?hl=en

Naive relaying or forwarding can/will clobber headers, modifying the from header will also set off reputation issues. If you forward you need to be using ARC headers. The milter is a total pain to set up, validate, and get working.

High volume sending also has stringent requirements. You can read all about it at that link.