Sending to localhost aliases and outside addresses with default a domain name using Postfix on Debian; working config

How to send to both local users (like root) from cronjobs, and to outside addresses with a default domain added that actually also passes SPF checks.

Related problems / information:

• Set up postmaster using Postfix
• Get postfix to forward root’s mail
• postfix /etc/mailname
• Postfix address masquerading

/etc/hosts

127.0.0.1 localhost mercurius mercurius.spacedomain.net

/etc/mailname

mercurius.spacedomain.net

/etc/aliases

postmaster:     root
root:   explorer@moonbase.org

/etc/postfix/main.cf

myorigin = mercurius.spacedomain.net
masquerade_domains = spacedomain.net
append_dot_mydomain = no
myhostname = mercurius.spacedomain.net
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = $myhostname, localhost, localhost.localdomain

TEST

echo test | mail -s test root