Sending email on Apple Watch without iPhone nearby

It seems I can’t reply to emails from my Apple watch on cellular, without my iPhone nearby.

I get the message “Emails cannot be sent from [EMAIL ADDRESS] unless your device is in range of your iPhone.

Have I set things up incorrectly somehow or, as I’m inclined to fear, is this not possible...?

If the latter, can someone explain how Apple could possibly justify omission of this function. Needless to say, I wouldn’t have wasted my money getting cellular connectivity had I known this was the case.

Many thanks.