This is how you can send a message from the commandline using the mailx (mailutils) package.
I just finished setting up postfix on a Debian 6 machine. All is peachy. However, when I checked my mail logs to see how things were going I noticed a number of entries for messages being sent from firstname.lastname@example.org to root. Checking root's mail led to messages with the following content:
/usr/share/sendmail/sendmail: line 880: /usr/sbin/sendmail-msp: No such file or directory