We had an interesting issue last week where a Visual Basic .Net application could connect to some mailboxes but could not connect to other mailboxes on Exchange 2010. The application uses MAPI to connect to the mailboxes. We were however able to access the affected mailboxes using Outlook and with the MFCMAPI tool from the application server. We moved the affected mailboxes to other servers and databases but the application could still not connect to the affected mailboxes. We then added our CAS Array name and IP in the application server’s host file and checked the Exchange RCA logs. We could not see any connections from the application server to…