When I installed the nagios port, there are come from this port some
sample config files.

In file 'command.conf-sample' thereis 'SAMPLE NOTIFICATION COMMANDS' section.
In this section there is sample command to notify by e-mail:

---cut here---
define command{
        command_name    host-notify-by-email
        command_line    /usr/bin/printf "%b" "***** Nagios 2.10 *****\n\nNotification Type: $NOTIFICATIONTYP
E$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATE
TIME$\n" | /bin/mail -s "Host $HOSTSTATE$ alert for $HOSTNAME$!" $CONTACTEMAIL$
        }
---cut here---

---cut here---
define command{
        command_name    notify-by-email
        command_line    /usr/bin/printf "%b" "***** Nagios 2.10 *****\n\nNotification Type: $NOTIFICATIONTYP
E$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time:
 $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$" | /bin/mail -s "** $NOTIFICATIONTYPE$ alert - $HOSTA
LIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
        }
---cute here---

In this two commands there is mistake "/bin/mail". In FreeBSD 6.2-Release
'mail' command is in /usr/bin/ location.

Fix: 

Aplly a patch to correct this wrong path to 'mail' program.