apple mail alias

I can receive mail from outlook, but apple mail won't verify or accept an alias - says it cannot verify, but receives mail from the alias no problem, so knows it is linked to it.

However, if I want to reply, applemail won't respond and brings up only the primary account.

Surely it is not beyond the capabilities of the app (or apple technicians and program writers) to set up the alias account - so we can actually use it??

How is this done? Am I missing something here? It seems such a basic feature - why wouldn't I be able to set up and reply from an account I already receive emails from?, I'm baffled I can't set up something this simple - or find out anything about it.

Any help greatly received. Thanks