I'm also getting an undeliverable mail to MAILER-DAEMON@localdomain, after running omv-snapraid-diff script. Here is the mail command in the script:
Since i don't have configured a secondary mail address, the variable $OMV_MAIL_secondarymail is empty and "-c" option is invoked without an argument. This seems to cause mail to send an additional mail to MAILER-DAEMON. Removing the "-c" option in the script, solved the problem for me.