can't send from @mac.com anymore in Apple Mail

Go to Mail Preferences -> Accounts and drop down the Email Addresses list.

Drop down the list of email addresses, I still see all 3 aliases. @mac.com / @me.com / @icloud.com but I've had the account since the beginning. Clicking @mac.com will put the checkmark there and make it the default.

If you click Edit Email Addresses it takes you to iCloud.com and you are likely to edit the defaults but you could create new alias.

Under Mail Preferences -> Composing there's a "Send new messages from" option which for me is Automatically select best account. Change it to your @mac.com alias or whichever one you prefer.

I do not know if brand new iCloud accounts only offer @iCloud.com and no longer offer the older aliases. But it would make sense.

I did discover if you have an iOS / iPad device you can turn these aliases on / off. On an Apple mobile device go to Settings -> (Your name at the top) -> iCloud and scroll all the way to the bottom and tap on Mail. There is an Allow Sending From and the list of all the aliases and you can turn them on / off and this will impact the iCloud account on macOS Mail after you quit and restart Mail on macOS.

Guess what? If it's disabled you can't change it here.

Finally, the best for last part. Here is the real solution. Go to iCloud.com and web Mail and click the Gear in the bottom left corner. Then click on the Composing tab at the top. Here is where you can actually turn on and off an email alias.

Come on Apple! Why is this option nowhere else but iCloud.com's web interface. This is truly bizarre. Maybe there is a good reason for it, but for the life of me, I can't think of it.