But as the user does not have a mailbox, it fails
Silently?
I am confused by the '< /dev/null'. This means send no email body just headers and attachment/MIME?
Yup, just need to send this attachment.
Well, I can't see the output as its ran by the daemon, I just try try to run as a user whom also doesn't have a local mb and get prompted which to create a new mb.
Just before I could look into Mike's reco, someone on the mutt list presented what just might be the working muttrc conf I needed.
-f /dev/null and 'set folder=/dev/null' looks like what I was after.
I should have the machine back up for testing tomorrow...