I often see and hear about this issue a lot and I wonder how other email companies cracked it. I use FastMail for instance and never have this issue, but I kno

I often see and hear about this issue a lot and I wonder how other email companies cracked it. I use FastMail for instance and never have this issue, but I know so many people who gave up on running their own mail servers at the small enterprise level because of stuff like this I often wonder how FastMail does not have these issues but others do. Is it a headers thing I wonder?