Is that the only thing on that box that is showing no availability? The first thing that comes to mind when that happens for me is the time is offset. An easy way to see if your server/agent time is off is to go to the HQ Health under the Administration Tab. Click on the Agent tab below and you can see the server time offset. Then you can use NTP or something to update the offending agent/OS.
BTW - for more insight on how HQ Health can help you - check out the May 6 HyperCAST where our own Scott Morrison from Hyperic Support will give a free tutorial. http://www.hyperic.com/demo/hypercasts.html to register.
After I do a fresh install both server and agent to 3.2.3 (not upgrade), the problem gone. I found that release 3.2.3 allow user configure smtp port in the configuration page. I think the problem had been fixed in release 3.2.3.