The story behind this is the following.
Like many other people, I have multiple Gmail accounts. Although I have multiple Gmail accounts, I only actively use a few of them and have the rest forward to one inbox so that I can get all emails in the same place instead of having to check dozens of accounts every day. In this particular case, I have Account A forward all emails to Account B. (This was setup a a few years back and it works perfectly — I’ve never had any issues.)
Because I have Account A forward to Account B, I don’t often actually log into Account A itself. Today, however, I logged into Account A and received an email from Google telling me about the “new” Gmail inbox. I put new in quotes because the inbox was introduced a few months back, I just haven’t logged into Account A since then… which is why Gmail sent me the email today. Google sends every Gmail user an email about the inbox changes when they log in, so this wasn’t too surprising and the legitimate email went directly to my inbox of Account A.
Once the email hit the inbox of Account A, it was forwarded to Account B as it should. What happened next was unexpected and an epic fail by Google: in Account B, Gmail marked the email as spam (the Google/Gmail email that was forwarded from Account A to Account B). In other words, Google marked their own email as spam — literally.
Now, there is some logic behind this action by Google. Because Account A forwards to Account B, Google’s spam filter probably suspected it could be possible Account A is sending unauthorized email to Account B (aka spamming) and thus marked the email as spam. However, that logic is blown to pieces when you consider the fact that Account A has been forwarding emails to Account B for years now and those emails were not marked as spam. So the fact that Google marked their own email as spam because they “couldn’t verify that this message was sent by google.com” is hilarious.