I have an Exchange account which I use IMAP connector for.

When I setup Zimbra, it worked fine. After a day, it stopped working.

Now, I can use Outlook and Windows Mail client with IMAP to connect to my Exchange server and I can connect using telnet xyz.server 143 and telnet xyz.server 993 but Zimbra keeps saying it cannot connect to xyz.server:143 or xyz.server:993; possible error not connected to internet.

What's the best way to troubleshoot this problem?