I had a long chat with an engineer from the Yahoo spam team about why Gmail was better, and this was his take: When Gmail started off in beta, the mail filters

I had a long chat with an engineer from the Yahoo spam team about why Gmail was better, and this was his take: When Gmail started off in beta, the mail filters were amazing. They were trained with a large corpus of Google's employee mail, skewing everything to a technical subject matter and a tightly knit network of contacts. When Gmail hit general availability, the quality of filters was not what average users expected. Things would get flagged simply because Grandma was not in the trusted circle of silicon valley. Eventually they bought Postini, both to boost corporate email business, but also to source a spam filter that had been battle tested across large consumer mail installs.