News: Google warnings of suspicious content

We're seeing an increase of failed deliveries to Google today which return this message:

Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information.

This relates specifically to content in your message. We don't know why there would be an increase, but we can offer a theory. In the past Google has suddenly marked a domain as suspicious which was frequently used in email contents. It could be an image hosting service commonly used to host images in a signature, a link shortening service (this did happen with bit.ly in the past) that is commonly used, or anything else really. If you get this message, consider all of the URLs in your email which might be buried in it's HTML source, and whether or not those might be contributing to the classification.