Management and Configuration > User Management > What is a sibling domain?

What is a sibling domain?

A sibling domain is an exact replica of a primary domain, in all respects except the name of the domain itself. The primary domain and the sibling domain must have the exact same mail host, email account lists, aliases, spam filtering settings, and so forth. For example, may be a primary domain while may be a sibling domain -- in which case, when the filter receives any message to an address at, it will treat the message exactly as if the message was sent to the same an address at The sibling domain can be added in the Domains section of the Control Panel.