Returned error: 'There was no endpoint listening at ... that could accept the message'

If you are getting the error like this one:

Then it's means that Greenrain Messenger was unable to connect to Post Office web service, that may happens because of firewall or proxy server restrictions in your local network.

 First of all you need to check if you're able to open the link from the error message in your browser, the link may be different depends on which Post Office you are trying to access (DHA on HAAD), the result should be like this(this is the example of access to HAAD Post Office):

if you're unable to open the link in the browser or you've got the result different from that shown on the screenshot, then you need to check your proxy or firewall settings to make it accessible.

In case if you were able to connect to Post Office in browser, but still getting the same error in Greenrain Messenger, you need to check proxy setting in Greenrain Messenger and make sure that you're using the same proxy settings in Greenrain Messenger and in Internet Explorer: