How to Fix: 550 5.7.350 Remote server returned message detected as spam

PaulAdebiyiWe had this issue and i just thought i'd put this out there.
Turns out the client was using Starlink internet on the dynamic IP plan.
A lot of the starlink dynamic IP Addresses appear to be blacklisted.
It appears through repeated sending from these blacklisted IP's a bad rep was created for the sending domain. (this is why sending from alternate domains works eventually the alternate domain would also build a bad rep) the combination of bad rep and blacklisted IP = failed emails. By changing the IP to IP4 it seems a new rep list is in play and so all emails were successful for about 2 months, and now the issue occurs on both ip4 and ip6.

The solution was to simply have a static IP on the internet connection , I'd also point out while this issue was at the main office of this client there could also be situations where staff are working from remote locations or home sending with a blacklisted IP and causing the domain name to get a bad rep, from what i can tell as long as the email is 100% clean it will still be delivered, but once you have a bad rep if anything else looks out of place the message will be blocked (as if you normally get 3 strikes but once you have the bad rep you only get 1)