Spectrum is the second largest ISP in the US and I have zero issues sending or receiving on my mail server, no relay in place

Spectrum in the US allows the customer to replace their provided router with their own equipment, so all they provided to me is the modem, after that I have my own router, so I can pretty much do whatever I want. Never had an issue hosting a mail server, sending/receiving works perfect.

We don’t run any WS on our internal infra, Linux/Unix in the enterprise space has a much higher usage than Windows, hands down

I had nothing but issues with RoundCube trying to patch the OS to the newest release, ended up switching mail providers entirely to Modoboa. Best of luck!