Thought I'd share what I learned after posting this request for help
I was using procmail to forward messages from certain mailing lists
to specific mail repositories, and it turns out that procmail can
forward and save messages in (at least) three different ways,
depending on how one has set it up:
1. Append to a file (the old "-mbox" alternative with MHonArc)
2. Write individual messages to a directory, with each message
having a unique file name (but not MH filenames)
3. Write individual messages to a directory in MH format (numbered
I was set up to forward in mode 2) above, which I had (mistakenly)
thought was MH format. (Thanks, Earl, for setting me straight.)
However, I just need to setup procmail for mode 1) above and I'll be
able to transition to the new ISP painlessly.
I'm sure all you procmail afficianados already know this, but for
other neophytes like me, I hope this helps.
P.S. Here's further information from the procmailrc man page:
Anything else will be taken as a mailbox name (either a
filename or a directory, absolute or relative to the current
directory (see MAILDIR)). If it is a (possibly yet
nonexistent) filename, the mail will be appended to it.
If it is a directory, the mail will be delivered to a
newly created, guaranteed to be unique file named $MSGPRE- FIX*
in the specified directory. If the directory name ends in
"/.", then this directory is presumed to be an MH folder;
i.e. procmail will use the next number it finds available.
When procmail is delivering to directories, you can specify
multiple directories to deliver to (procmail will do so