Try the following:

• Restart Xeams and watch Xeams.log to confirm if IMAP and POP3 servers did not run into any problem starting up
• Use Telnet to confirm TCP/IP port is open. You can do that by opening an SSH session to QNAP and typing the following command:
  
  telnet localhost 143
  
  Change the port 143 to the appropriate port on your end. You should see the IMAP  banner. Is not, something is wrong starting the server. Contact our support via email in that case.
• Change ImapConversation.log to confirm clients are connecting from other machines.

Contact our support via email if you're unable to figure it out from the logs.