

Sounds like a management problem. Have these issues been brought to them/have they done anything about it? ITIL is a good foundation to follow for how things should work. Define a service catalog, SLAs, escalation process, etc… in that escalation, at a certain point management should be made aware. If they are and do nothing, you need to look for another position, and if you can’t, find a way to deal with a broken system. If someone sends a ticket back to me after I have written clear documentation, I reply with the document asking what part they didn’t understand. No one likes getting throw under the bus, or doing the throwing, but if the system is broken, I start copying additional people on those kickback emails. CYA, always.
If their staff weren’t incompetent and understand SPF records, DKIM and DMARC, they wouldn’t be flagged as spam. But when morons who don’t understand the tech fail at using it, it’s not the systems fault for identifying it as garbage. Source- me, 25 years supporting email systems.