Typical examples of this property value: mail.domain.com ([18.104.22.168]), MYSERVER (mail.domain.com [22.214.171.124]), etc. There is no strict format for this value.
This value does not necessarily denote the original host which sent this message. Instead, it contains the name of the host which delivered the message to the host which added this time stamp to the message. If the delivery chain consists of many servers,server simply denotes the previous link in the chain.