icloud alias not working on mac mail on computer

I have a domain email hosted by icloud and I also use an alias icloud email. The icloud alias works in icloud and on my iphone, however it isn't completely working in mac mail on my computer using 14.7. The alias mail folder is there and I can read and receive emails in mac mail on my MacBook Pro, however I cannot send an email in mac mail on my macbook pro from the alias address.

It used to work on my previous macbook pro but I'm not sure if the ability to send from the alias email has ever worked on the new laptop. I have tried going through the motions of setting up another email account in mac mail on my computer but it only results in the one domain email address being listed in the settings, and the alias inbox being there, but I still can't send emails from that alias icloud address. How do I resolve this?