This morning there was a problem with the implementation of Google’s malware filters. Gmail’s spam engine uses those filters (among hundreds of other signals) to help protect our users from malware, and so between 6:00 a.m. PST and 8:00 a.m. PST, we mistakenly sent some legitimate mail to people’s spam folders.
These trouble were being faced by many Gmail users yesterday, and Gmail now says that they have fixed the issue and have moved all the messages, that it mistakenly treated as spam, back to inbox, but you may still find some emails on your spam folder too.