Some special processing occurs if the ruleset zero resolves to an IPC mailer (that is, a mailer that has [IPC] listed as the Path in the M configuration line. The host name passed after $@ has MX expansion performed; this looks the name up in DNS to find alternate delivery sites.
The host name can also be provided as a dotted quad in square brackets; for example:
The host name passed in after the $@ may also be a colon-separated list of hosts. Each is separately MX expanded and the results are concatenated to make (essentially) one long MX list. The intent here is to create fake MX records that are not published in DNS for private internal networks.
As a final special case, the host name can be passed in as a text string in square brackets: