Hi,
since the update of mdadm to version 4.0 the email notifications does not work:
mdadm --monitor --test --oneshot --scan --config=/var/etc/mdadm.conf
sh: /usr/lib/sendmail: not found
sh: /usr/lib/sendmail: not found
Patch /utils/mdadm/patches/300-correct-sendmail.patch which fixed the sendmail path in mdadm has been deleted in commit 6ac84e7a, maybe this is the reason?