Comment # 1 on bug 1169563 from Neil Brown

Thanks for the report.
Can you please try adding the line
  After=multi-user.target
to /usr/lib/systemd/system/mdmonitor-oneshot.service
in the [Unit] section, then see what difference that makes.
I'm hoping you won't get the sendmail error messages, but will get the email
about the degraded array.