Let's start with the cert mismatch, go through the article below, just run the get and not the set against each cas. Possibly one of your cas servers Web services was not set correctly and it's hitting the bad cas intermittently. Outlook 2007 security warning: "The name of the security certificate is invalid or does not match the name of the site" http://support.microsoft.com/kb/940726James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com

James is right. You may also chage another certificate that contains the names of your web-service urls.Fiona Liao TechNet Community Support

Running on Terminal Services 2003 R2 sp2, using Outlook 2007 to exchange 2010. Of the 20+ users only a couple of them are getting one and/or two prompts: 1) Prompt to authenticate to Exchange 2) AutoDiscovery cert name does not match prompt. prompts are not consistent with start up or action, can go days without them happening. User can be just sitting viewing the Inbox when they pop up. Auto Discover cert is correct and is not the Self-signed from install. Have deleted email profile(s) for the users, still returns. Any ideas what to look at next? Could Remote Desktop Client play into this, one user's local PC is Windows 2000 computer?

Let's start with the cert mismatch, go through the article below, just run the get and not the set against each cas. Possibly one of your cas servers Web services was not set correctly and it's hitting the bad cas intermittently. Outlook 2007 security warning: "The name of the security certificate is invalid or does not match the name of the site" http://support.microsoft.com/kb/940726James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com