The Gmail team has taken some time to explain why messages may end up in the spam folder. Of course, as they ever so kindly point out, “the accuracy of our spam filter is one of the key reasons they love Gmail.” But that being said, we are fairly certain that everyone has seen the spam filter slip up a time or two and deposit a real (and expected) email message into the spam box as opposed to the inbox. Anyway, moving forward, should you find something along that line — you will now have some additional information to make sure it doesn’t happen again in the future. The details from the Gmail Team include how each spam message will have an explanation at the top of the message. The explanation will include details such as whether the message may contain a virus or bad link or if you previously marked something from that sender as spam or even if it was deemed to be similar to previous spam. Bottom line, the filter still has the potential to miss or mix something up, however you will at least have a bit more to go on in the future.
Via [Official Gmail Blog]