Sure. Highly successful even, I would say. I can deliver to Microsoft and Google.

Not sure though what the magic ingredient is. I've had the IP address for 7 years before I decided to use it for mail, after one quick mail to Cisco's Talos stuff everything was fine. Software is Mailcow. Hosted at Hetzner in Germany.

And still, I cannot deliver to T-Online, so there's that.