To manage a mail resource under service domain, go to the Mail Info menu and chose the third-level domain from the drop-down box:

Note that mail domain shows both third-level domain (health.test4.psoft) and service domain (test4.psoft) it is created under. If you create a mailbox under the third-level health.test4.psoft, it will look like user@test4.psoft.

test@test.com, another user can't create mailbox named test@test.com.test@test.com, another user can't create mailbox named test@test.com.