Glen Lee Edwards asked,
| What I need to know is, is there some way to have procmail pipe one letter
| at a time to the list program, wait for it to execute or fork, and then
| send the next letter? The list program is written to only handle one
| letter at a time.
Well, there are several approaches. If the messages are in separate files, a
simple shell loop can make procmail iterate over them, processing only one at
a time, or for that matter you can skip procmail and let the list program
iterate over them instead. Alternatively, you can use global lockfiles in
the procmailrc code so that only one instantiation of procmail at a time can
invoke the list program.
If the messages are concatenated in a single file, you need to use formail,
not procmail, to separate them and invoke the list program. Check its -s
option (and, if appropriate, -d, -e, and -m; -n will not apply this time).