[Mimedefang] value of $RelayHostname

[kamal]

In filter_end, I want to know whether reverse DNS lookup had succeeded.
So I check the value of $RelayHostname. The man page says that it "May
be undef if host name could not be determined". But in my case, (when
reverse DNS fails) the value is IP address surrounded by square
brackets. Of course, I can check for this, but is this behaviour
guaranteed? Can anyone please explain what determines the value of
$RelayHostname?
Thanks.