Guy Geens suggested to Gilles Carmel:
| # If the command contains an error, the subject header if empty
| SUBJECT=`formail -zxSubject:`
|
| # If we have a subject, the command was processed correct, so we send the
| # mail
| :0
| * SUBJECT ?? .
| | $SENDMAIL -t
You don't have to run formail there. This will be much less load:
# Check if there is a subject line and that it contains at least one
# non-blank character, and if so send it on:
:0 # brackets enclose caret, space, tab
* ^Subject:.*[^ ]
! -t
Again, it presupposes that there is a way for procmail to see Gilles's
outgoing mail.