Here's an idea: include $HOST as well as $$ in the name of the logfile. Then
you can't possibly get a conflict.
I didn't mention before that if two hosts have the same log _directory_,
probably only one of them handles mail for that network. Here, each user
has a particular host that handles his/her mail. Mail to that user at
another host is forwarded to the users "oficial" mail host, and only there
would procmail get called, even though the .forward file is visible on
many hosts and the MUA (reader) can run on many hosts.
Well, that was confusingly worded. Put another way, we have many hosts
that handle mail, BUT for a particular user, only ONE host will check that
user's .forward file to call procmail (or whatever).