Did you receive more spam than usual in your Gmail account at the end of June? Or did you receive more spam IN YOUR INBOX than usual? It might have been due to this. Google just released a root cause analysis of an issue from June 30th, where "Google's email delivery service was targeted in what we believe was an attempt to bypass spam classification." Sounds like the issue resulted in email delivery delays and some messages not getting spam filtered properly. Find more details here.
I *think* this is the same issue that Ben Schoon from 9to5Google is reporting on here.
Hey good senders, this is just another reminder that sometimes ISPs/webmail providers have bigger things to attend to, beyond whatever our problems are. It's good to remember that we may not be the only problem on a provider's plate. And let's not forget that there are lots of bad guys out there trying to send BILLIONS of spam messages every day. You'd never believe the amount of spam a large webmail provider like Gmail or Yahoo Mail (Verizon) or Microsoft OLC (Hotmail) are forced to process or reject every day. The unwanted junky stuff made up around 45% of internet email traffic as of March.