Error using environment variable for email setting
I think you're getting a few conventions mixed up.

Using an @ is typically reserved for aliases https://docs.craftcms.com/v3/config/environments.html#using-aliases-in-control-panel-settings

What you're probably looking for is environment variables in your .env file.

You could set:

EMAIL_HOST=myhost
EMAIL_PORT=myport
EMAIL_USERNAME=myusername
EMAIL_PASSWORD=mypassword


Then from Settings->Email in the control panel, you can reference them with $ https://docs.craftcms.com/v3/config/environments.html#control-panel-settings

$EMAIL_HOST
$EMAIL_PORT
$EMAIL_USERNAME
$EMAIL_PASSWORD