Comment 2 for bug 379882

The monitoring facility of mdadm is very important to get notice if somthing goes wrong with your raid.

So you can replace disks etc. ahead of a total failure.

To send out notifications mdadm needs a sendmail command (MTA mail transport agent ).

To deliver mail localy you need a mail delivery agent (MDA)

Things like exim, postfix open network ports and are large and not easy to configure, but of course provide the sendmail command and delivery.

If none of the large MTAs is installed, yes mdadm should pull in a replacement like esmtp + procmail but no postfix on desktops.