I am having this issue as well. Very troublesome. I have all SW updates installed, all apps are up to date. I do use Swiftkey3 but have switched back to the Samsung keyboard, restarted and tried again, nothing. To make sure I am on the same page I do not have any email accounts setup in the Samsung default email client. I changed the quick command settings for the @ to open Gmail. When I type the @ in quick commands it says opening Gmail then returns to the home screen and does nothing else. I have tried using the Samsung keyboard, unchecking sync contacts. I am not using the 2 step verification on this account. When it was still set to open the native app with Swiftkey as the keyboard it worked fine so I don't think (at least on my phone) that the 3rd party keyboard is the issue. Not sure what else to try. I don't want to add my gmail in the native app and get double notifications just to use the quick command.