I've got a bash shell script file which is executed as a service under root.
/usr/bin/mutt -s "subject" -a report.zip firstname.lastname@example.org
"Error sending message, child exited 67 (User unknown.)"
When this is not ran as a service or from the command line, the message get send and I don't get this error. I also echoed "whoami" which resulted in "root".