User configurable mail filters. Again.
That's good news. 
Just for information, if your email client is set to apply the filter "on arrival", this probably means "when the message status is unread". So if you'd read it in webmail (I'm assuming you didn't) then openied the client the filter might not have triggered.
Just thought I'd mention that in case prior reading leads to thoughts that the filter isn't working.
That was a very good thought.
I'd fiddled with the filters so much and been back and forth between Mail Client and Webmail I'd somewhat lost track of what, how, where and why... (You get my drift).
This AM after reading your note, I did a few test posts and I find on my system, opening a posting in Webmail doesn't interfere with the filter, so opened or not I never saw the posting mail client side thereafter.
AIUI. The transport part of the mail client setup, with the particular filtering I'm using, looks at the mail server side and if it matches the filter proscription settings, it doesn't even get downloaded, but just deleted server side.
That said, as just a mere user of this stuff... I could of course be misinterpreting the info given to me about the mail client...
But however it works, it does do the business.
Thanks again for that good thought.
Ax