Update:
I was told to use
:0
* ! already-processed-flag
| program | $SENDMAIL -oi myname(_at_)mysite(_dot_)com
I verified from the command line
program < message > message.modified
diff message message.modified
that if stdin is a "normal" message, then stdout is the message with the
extra header added and no other changes. So I wonder whether a better
approach would be
:0 fhw
| program | $SENDMAIL -oi myname(_at_)mysite(_dot_)com
Actually, the recommended approach does not seem to work. I added the 'c'
flag because I was suspicious. Result: I get two log files for each
message, but the only copy I ever see is the unmodified one.
I did a lot of experimentation. If I followed the directions exactly, I lost
all mail.
If I added the 'c' flag, i.e.,
:0c
| program | $SENDMAIL -oi myname(_at_)mysite(_dot_)com
then I only received the unmodified copy.
If I used the fhw flags, everything worked as expected.