I think it's worth to mention that emails on special domains, e.g.
[email protected],
[email protected],
[email protected], etc. may not be used.
What is more confusing is that the setup script will process them fine and create an appropriate user, but webUI won't let you in showing error that the email is not valid. In the container log the error will be more meaningful and say "The domain name ... is a special-use or reserved name that cannot be used with email."
Another concern is that, if such email was set, and pgadmin4 was initialized, it will not be enough to simply change PGADMIN_DEFAULT_EMAIL and PGADMIN_DEFAULT_PASSWORD variables and restart the container. Since setup process is executed only during the first container start, it will be necessary to remove the contents of the /mnt/user/appdata/pgadmin4 directory.