To give all your users an alternate email address at another domain, add the domain as a user alias domain. Adding the domain solarmora.com as an alias to example.com, gives bob@example.com another email address at bob@solarmora.com.
If the domain you're adding has its own set of users, add it as a secondary domain. Users can have email addresses and accounts at their domain.
Learn more about adding domains and limitations with multiple domains.