In order to improve security, Google recently added an option that blocks what they consider “less secure connections”.

By less secure connection, Google means that a third-party software trying to login using your username and password is more vulnerable than one using the more complex OAuth identification process.

The e-mail piping addon uses this method to login and that’s the reason why, with some accounts, the piping won’t work. Don’t worry though, this doesn’t mean that the addon is not secure. In fact, most e-mail clients use the same method: Microsoft Outlook, Mozilla Firebird, Mailbird…

In order to solve this problem, you need to allow “less secure connections” in your Google Apps settings. The process is detailed in Google’s documentation: https://support.google.com/a/answer/6260879?hl=en