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, yourcompany.com may be a primary domain while yourcompany.net may be a sibling domain -- in which case, when the filter receives any message to an address at yourcompany.net, it will treat the message exactly as if the message was sent to the same an address at yourcompany.com. The sibling domain can be added in the Domains section of the control panel.