Domain example.com has alias domain_alias.com.
/usr/local/psa/admin/bin/mail_auth_viewutility shows that there are some mailboxes under domain_alias.com.
Unable to access mailbox@domain_alias.com
Plesk does not allow creating mailboxes under domain aliases.
Mailboxes of the primary domain are used. If the Mail service option is selected, the mailboxes created under the primary domain will also be available under the alias. To enable users to read mail sent to mailboxes under the alias, Plesk redirects it to the corresponding mailboxes under the primary domain.
More details are available in Plesk Administrator Guide .