A frequent question I receive is how to monitor the availability of BPOS and how to access BPOS service notifications.   There's a few good techniques and resources out there.  In my experience, BPOS availability has been great and I've only had one issue since August 2008 when I started using it. It cleared up within a minute or two.  Here's a couple of good tips:

Monitoring BPOS

There's a few different ways to monitor your BPOS service.  If you own a monitoring tool such as Solar Winds' IPMonitor (http://www.solarwinds.com/products/ipmonitor/) you can easily configure fairly sophisticated queries against your various BPOS service URLs such as SharePoint sites or OWA.  I generally configure some alerts againsts my SharePoint sites and OWA.  In the use of your BPOS service you also rely on either your own DNS hosting infrastructure or perhaps a third parties.  I set up QoS monitoring on all my DNS servers - remember if DNS is not working properly mail flow, your website and other services may not function as you have designed.  You can also probably monitor SMTP by configuring a dedicated user (consuming a license) and using an SMTP monitoring rule that can be configured to support authentication (these are available in the IPMonitor product). 

BPOS Service Notifications

BPOS Service Notifications are another great way to keep in touch with Microsoft annouced service issues or maintenance. 

North America Microsoft Online Services Standard RSS Feed https://rss.microsoftonline.com/

APAC Microsoft Online Services Standard RSS Feed https://rss.apac.microsoftonline.com/

Europe Microsoft Online Services Standard RSS Feed https://rss.emea.microsoftonline.com/ 

Message Filtering RSS Feed http://rss.messaging.microsoft.com/