Filters to tag newsletters is a visually helpful tool I make use of in GMail. To my dismay and frustration a couple of days ago, I found I was unable to save new newsletter e-mail addresses to a GMail filter I have set up.
Turns out the solution is simple enough:
Clear cookies.
Clear cache.
A heads up. I found I needed to wait something from a full minute, to possibly several minutes after doing the above steps. During that time when I clicked the settings icon in GMail I got zero response from the action. It was enough to send me back to the Google help forum to describe the new issue, but when I returned to GMail and clicked the settings gear again, everything worked as expected.