With the introduction of Exchange 2010 more environments are deploying HLBs (Hardware Load Balancers) to provide high availability. With the deployment of these device your users can expect a more stable and enjoyable experience with Outlook. However before you deploy your HLBs, testing of the features that will be behind these devices.
What should you test for?
Autodiscover – Can you create a profile automatically with Outlook? Can you run a Test Email Autoconfiguration successfully?
Out of Office – Can you set this without getting an error?
OWA – Test an Exchange 2010 and 2003 mailbox (for redirection purposes)
ActiveSync – Test an Exchange 2010 and 2003 mailbox (for proxy purposes)
Outlook Anywhere – Test an Exchange 2010 and 2003 mailbox (for proxy purposes)
If any of these fail, fix the issue before adding the HLBs to the mix. Then once all issues are resolved, then insert the HLBs. If there are any errors now, you know that your HLB is most likely the cause. Tweak your HLB rules until it works.
One last consideration, if you are using TMG/UAG, test your rules before and after as well.