January 31st, 2009 | Published in Gmail (Google Mail)
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.
We're working to roll out an automated fix to put these legitimate messages back into your inboxes, and we expect this to happen within a day. In the meantime, if you were expecting a critical message this morning, please check your spam folder. (We tune our spam filters well enough that ordinarily you should never have to check your spam folder.)
We're very sorry for the inconvenience. We'll update this post as we have more information to share.
Update (2/1): We've rolled out a fix that has restored these messages to most people's inboxes, though to be on the safe side we'd still recommend that you check your spam folder if there was a critical message you expected to receive between 6am and 8am PST on Saturday.