FWIW, I have been using the companyname@mydomain.com auto-alias for many years now and I've never had it challenged nor rejected by a human or a machine.

I’ve also been doing it for quite a few years, and I think I had it rejected by a machine once, and I had it questioned by a human once.

I’ve had way more problems from systems that think TLDs are two or three characters (which has never been true).