I know it sounds obvious, but I've owned several Amcrest cameras for 5 years and I still wasted an hour this morning trying to figure out why some of them stopped sending email over the last two days. I also couldn't access any of them in the Amcrest View Pro app on Android. Everything was finally resolved by doing a Manual Reboot on each camera.
I got comfortable to do the (unfortunately) all-too-familiar troubleshooting steps, but nothing worked. All my email tests were failing. Finally I went to the System-->Auto Maintain screen and clicked the Manual Reboot button. After each camera restarted, everything was back to normal. When these cameras work, they work well and have great features. But I always question if they're worth the frustration.