Gmail: That's spam, and here's why
Opening a given e-mail message marked as spam now gets you an explanation of why Gmail considers it junk.
Wondering why a certain e-mail was dumped into your Gmail spam folder? Google will now clue you in.
As of yesterday, Gmail users can select any message banished to the spam folder and see a "Why is this message in Spam?" notice near the top. The notice will display a brief explanation accompanied by a "Learn more" link to a page describing the many reasons certain messages are considered spam.
A look at the e-mail in my own Gmail spam folder revealed a variety of explanations.
For one e-mail that claimed to be from YouTube but clearly was not, Gmail said that "our systems couldn't verify that this message was really sent by youtube.com." Another e-mail hawking phony Adobe software was flagged as spam because "many people marked similar messages as phishing scams, so this might contain unsafe content."
A third e-mail that looked like it was sent from Twitter was considered junk because "similar messages were used to steal people's personal information." And a batch of other messages were flagged mainly because they were "similar to messages that were detected by our spam filters."
I've found that Gmail does a pretty good job of separating the junk from the non-junk, so I usually don't bother to scan through my spam filter. But Google's helpful spam explanations can alert you to specific unwanted e-mails and tell you what to do when you do get spammed.