$value = apply_filters( "pre_update_option_{$option}", $new_value, $old_value );

$value = apply_filters( 'pre_update_option', $value, $option, $old_value );

<?php
/** Plugin Name: (#16621) Send mail after Option Value changed */

add_action( 'plugins_loaded', '166221reactMail' );
function 166221reactMail()
{
    // @TODO Add you option names (valid triggers) here:
    $valid = [
        'your',
        'option',
        'names',
        'here',
    ];

    // Attach each filter: Precise targetting option names
    foreach ( $valid as $name )
    {
        add_filter( "pre_update_option_{$name}", function( $new, $old )
        {
            # @TODO Adjust check/validity of mail trigger here
            if ( $new !== $old )
            {
                # @TODO uncomment the following line - see note below
                // add_filter( 'wp_mail_from', '166221reactMailFrom' );

                # @TODO Adjust values for wp_mail()
                wp_mail(
                    'To@example.com',
                    sprintf( 'Change notification from %s', get_option( 'blogname' ) ),
                    sprintf( 'Value changed from %s to %s', $old, $new )
                );
            }

            return $new;
        }
    }
}

// @TODO Change "from" Name to make creating inbox rules easier
function 166221reactMailFrom( $from )
{
    remove_filter( current_filter(), __FUNCTION__ );
    return "foo@bar.com";
}