Jim has been having issues with his email bouncing back on him. It happens with four different hosts. Leo says that the ones that are bouncing are from older ISPs like Earthlink. Gmail adheres to all modern standards, and it may be that those hosts are misconfigured, especially since they all started to happen on the same day. It sounds like Gmail has a new standard protocol enabled that is preventing these carriers from going though to and from Gmail. It'll likely be fixed soon, but it's likely a new Gmail protocol.
The chatroom says that there are reverse DNS rejections going on. Here's a thread on it - https://community.spiceworks.com/topic/2109221-problems-with-earthlink-mindspring-and-reverse-dns-rejections