https://snapcraft.io/blog/how-to-preserve-old-software-with-snaps

Now that you have installed the required components to enabled Nagios to send mail notifications, you will need to download two configurations files from your Nagios Server (using FileZilla and edit them using Notepad++ – if you need to understand how to do this, please review part 2 of my installing Nagios for Exchange series).

These files are:

1. commands.cfg
2. contacts.cfg

Which are located in:

/usr/local/nagios/etc/objects

Open the commands.cfg file in Notepad++ and modify the following two lines:

# 'notify-host-by-email' command definition
define command{
    command_name    notify-host-by-email
    command_line    /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /bin/mailx -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$
    }

# 'notify-service-by-email' command definition
define command{
    command_name    notify-service-by-email
    command_line    /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /bin/mailx -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
    }

To look like the following:

# 'notify-host-by-email' command definition
define command{
    command_name    notify-host-by-email
    command_line    /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /usr/bin/mailx -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$
    }

# 'notify-service-by-email' command definition
define command{
    command_name    notify-service-by-email
    command_line    /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mailx -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$
    }

In essence you are changing the /bin/mail part of each command to /usr/bin/mailx

When you are done, save the file and then using FileZilla upload it back to your Nagios Server.

Next open the contacts.cfg file and change the “email” property to that of the account where you would like the notifications to be sent – see below

define contact{
        contact_name                    nagiosadmin        ; Short name of user
        use                                generic-contact        ; Inherit default values from generic-contact template (defined above)
        alias                           Nagios Admin        ; Full name of user
        email                           administrator@prepad.com    ; <<***** CHANGE THIS TO YOUR EMAIL ADDRESS ******
        }

When you are done, save the file, and upload it back to your Nagios server.

To finish up the configuration, from a PuTTY session type in the following command:

sudo /etc/init.d/nagios restart

Source: Configuring e-Mail Notifications in Nagios Core… – telnetport25.com