The problem occur when I was trying send a email to someone. After I sent out a email, a windows pop up told me that unable contact the update and licensing server. refer picture 
After that emclient change it to offline mode.
What is the issue cause?
Hi Chen,
there is a problem with eM Client’s connection. There could be a third-party app blocking it.
Make sure you have an exception in your windows firewall and anti-virus software for eM Client.
Also please let me know if you can access our licensing server at http://licensing.emclient.com/ in Internet Explorer and if you are using any proxy server to connect to the internet.
Regards,
Olivia