Sean Straw wrote,
| ... it seems as if [those who need to] should be able to just write a shell
| script for launching your mailer of choice, and in that shell script create
| (and on exit, delete) a global mail lockfile (likewise, it should actually
| check for one before creating it). Then at the top of your procmail, check
| for that lockfile - if it exists, defer processing of the new message.
| I believe there was a concise solution posted a few months back, perhaps
| some affected individual could rummage through the archives for it?
It's in the examples/ subdirectory of procmail's source code. There are
a sample procmailrc and a sample MUA wrapper that both rely on the same
global lockfile.