The mailbox and its alias have different domain parts (i.e. mail domain and its alias) and the same mailbox names. For instance, example.net being a mail alias to example.com, every mailbox on example.com will have a counterpart on example.net, specifically firstname.lastname@example.org will be an alternative mail address to email@example.com.
Mail domain aliases come with regular domain aliases only. You can't have a mail domain alias without a regular domain alias.
Like most other features, mail domain alias can be enabled or disabled in your hosting plan. To have it enabled, change plan or contact your hosting administrator.
You can create mail domain aliases for ALL your mailboxes on the primary mail domain, rather than for an individual mailbox. To enable mail domain aliases:
1. On your control panel home page, click Domains.
2. On the page that appears, click Add to add a totally new domain alias or Edit to use your domain alias with mail service:
3. On the next page, make sure to check the New Mail Domain Alias box. Then click Submit.
4. On your control panel home page, click E-Mail to check whether Mail Domain Alias has been activated:
Note that on the Mail Settings page you can create mail domain aliases only based on existing domain aliases.