You can have multiple inbound domains on your Exchange account, but only one of them can be set as default. The default domain is the base domain name for actual email addresses assigned to mailboxes. All other domains are added as aliases. 

When you register your first domain name through CONTROL PANEL, it is automatically set as the default. After you add other domains, you can switch the default domain if necessary.

To change the default domain name:

  1. Log in to CONTROL PANEL and navigate to Services > Domains
  2. Select the Default option for the domain name that you need to be the default.
  3. Click Save Changes.
  4. You will be prompted for confirmation. You need to decide whether to set the new default domain name as the default reply-as address for all mailboxes, distribution lists and public folders on your account. If you click OK, default reply-as addresses will be updated to the new default domain name.

    Note: the Primary Email Addresses can be updated for individual mailboxes. Navigate to Services > Mailboxes > Action > Edit Email Addresses and updating the Primary address there.