external monitor

What is the expected output of http://x.y.z.com/BroadcastState/Default.aspx?obj=Smtpbackendserver if the SMTP server is healthy?

yes

That means you only need an HTTP monitor.

You can create a custom monitor for the SMTP server, specifying the following:

Type: HTTP
Send String: "GET /BroadcastState/Default.aspx?obj=Smtpbackendserver HTTP/1.0\r\nHost: x.y.z.com\r\n\r\n" (No quotes)
Receive String: yes
Alias Address: IP address of x.y.z.com
Alias Service Port: 80

.

https://devcentral.f5.com/codeshare?sid=413

Other useful links:

K00764435: Expected behavior of an SMTP monitor https://support.f5.com/csp/article/K00764435

K7444: Requirements for external monitor output https://support.f5.com/csp/article/K7444

Not sure why you need an external but here's one based on /config/monitors/sample_monitor anyway:

!/bin/sh


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 by offering the updating of files via the Internet.

 @() $Id: //depot/maint/bigip11.6.2/tm_daemon/monitors/sample_monitor1 $




 these arguments supplied automatically for all external pingers:
 $1 = IP (::ffff:nnn.nnn.nnn.nnn notation or hostname)
 $2 = port (decimal, host byte order)
 $3 and higher = additional arguments
 
 $MONITOR_NAME = name of the monitor
 
 In this sample script, $3 is the regular expression


 Name of the pidfile
pidfile="/var/run/$MONITOR_NAME.$1..$2.pid"

 Send signal to the process group to kill our former self and any children 
 as external monitors are run with SIGHUP blocked
if [ -f $pidfile ]
then
   kill -9 -`cat $pidfile` > /dev/null 2>&1
fi

echo "$$" > $pidfile

 Remove the IPv6/IPv4 compatibility prefix 
node_ip=`echo $1 | sed 's/::ffff://'`

 Using the nc utility to get data from the server. 
 Search the data received for the expected expression.

echo "GET /" | /usr/bin/nc $node_ip $2 2> /dev/null | grep -E -i $3 > /dev/null
echo "GET /BroadcastState/Default.aspx?obj=Smtpbackendserver HTTP/1.0\r\nHost: x.y.z.com\r\n\r\n" | /usr/bin/nc ip_addr_of_x.y.z.com 80 2> /dev/null | grep -E -i "yes" > /dev/null

status=$?
if [ $status -eq 0 ]
then
 Remove the pidfile before the script echoes anything to stdout and is killed by bigd      
    rm -f $pidfile
    echo "up"
fi

 Remove the pidfile before the script ends
rm -f $pidfile

You need to add the monitor name, host name and IP address where necessary.