What expression can I use?

[lisajeanrieken]

I want to create an expression for Zabbix so that it will fire off a trigger when it is unable to communicate with the host's server.
I've tried this:

NAME OF TRIGGER:
Lisa: Unable to communicate with Server
(Lisa being the server name)

EXPRESSION:
{lisa.myip.co.za:service_state[SMTPSvc].nodata(400)}=1

The =1 doesn't seem to be working tho. It does send an alert when the server is down, but if I go to History and then Events, every seven minutes, it says ON, and a second later, it's OFF. Example

2007.Apr.02 16:13:24 Lisa: Unable to communicate with Server OFF Disaster
2007.Apr.02 16:13:24 Lisa: Unable to communicate with Server ON Disaster

Can someone pls tell me what the correct expression is??
Thank you!

[Alexei]

Make sure that interval for the service_state[] is much smaller than 400 seconds. I would also suggest using of agent.ping for nodata().

[lisajeanrieken]

So could you please give me an example of what it would look like?
How much smaller than 400?
What should the expression look like exactly?

[bashman]

An example:

http://www.zabbix.com/forum/showthread.php?p=59540