Good Morning -
I am a full-time Senior Network Engineer based out of the US with vast experience in routing, switching, and network security.
In your instance, I have run into this issue many times while working in a NOC in which we provided managed services to multiple clients.
We will need to identify why your traffic is being marked as spam. There could be underlying issues within your network (malware, viruses, etc.) that could be causing your IP address (domain name) to be flagged. You may also have a comprised email box.
My first step will be to do a network discovery. I would like to work with you to determine how your network is laid out, who your internet service provider is, etc. From there, we can investigate possible root causes of this "flag" you are obtaining.
Once we address any underlying issues, we can then submit an MX whitelist. That usually takes a day or two to replicate.
Look forward to working with you.
Best,
Mohammad