Outlook Connectivity issues

Disable the Server Certificate check in I.E. as a quick fix
 

1. Click Tools | Internet Options
2. Click Advanced tab.
3. Scroll down to "Security settings".
4. Deselect Check for Server Certificate Revocation.
5. Click OK. 

Possible Causes & Alternate Fixes

1. Proxy is caching an old certificate

• Clear the cache on the proxy

2. Bad Proxy Setting

• In your browser go to Tools - Internet Options - Connections and click the LAN settings button
• Confirm proxy setting for your LAN

3. Proxy is accidentally turned on

• Go to Start on your computer and click on Run
• Type regedit and click OK
• In the registry editor navigate to the registry key "ProxyEnable" under HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings
• Make sure its value is 0. If not click on the name ProxyEnable and set the value to 0 close the editor

4. Outlook lack security rights to install certificate

• Check what user is being used to run Exchange. (Local Machine may not be sufficient)
• Replace user with an admin user, or at least a user with certificate install rights

5. Bad Plugin Install

• Uninstall and reinstall the plugin

6. Time Mismatch

• Compare local machine and/or BIOS clock with a machine that's working
• Change to match the working machine. (they need to match Salesforce.com clock)

7. VPN Interference

• Disconnect from VPN
• Reinstall the Certificate