Today was the 8th time my boss asked me: "I haven't gotten any new mail in
a few hours, is there a problem with our mail server?" Of course I
immediatly told him to hit the clear button to clear his current filter.
And it worked.

I recognize that I have yet to think of the perfect solution to this. At
one point, you want to remember any user setting, including the filter. At
another, it is causing a noticible problem: people don't remember to clear
their filters. Now, sure, it's a people problem at the heart... but isn't
software suppose to be designed so people can use it in the simpliest
mannor possible?

The best idea I can think is a big red "!" that lets them know they have a
filter set. Or maybe a contrasted message, someplace. Just so they know
they have a filter when they enter a folder. I don't have a perfect
solution. My boss is pretty technical, and yes, he's VERY ditzy and
forgetful, but that doesn't stop it from being a problem that causes him to
come to me (the helpdesk) for support.