After upgrading from Outlook 2007 to Outlook 2010 we were unable to connect to pop servers on port 110.
A network monitoring utility (SmartSniff) showed that Outlook 2010 starts it's communication with a pop3 server by issuing a CAPA command. This command is supposed to retrieve a list of the server capabilities but when issued on the our network the TCP connection is immediately dropped when CAPA is sent. This can be seen by starting a telnet session with the pop server and sending the CAPA command.
Further investigation showed that Cisco routers can be configured to block invalid pop3 commands and that the router considers CAPA to be an invalid command.
The Cisco router has probably been configured to do this by running a variation of the following:
ip inspect name inspection-name pop3 [alert {on | off}] [audit-trail {on | off}] [reset] [secure-login] [timeout number]
We should just need to turn off the inspection of pop3 packets to resolve this problem.
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