My life was changed the day a friend showed me how to view source.

Looks interesting, definitely will give it a go.

Can anyone suggest other mail clients worth trying out? I'm tired of Mail.app these days.

Just thinking aloud here. Could that problem be avoided by putting the tooltip element outside of the thing it is tipping, and then using a CSS selector like ".tipped-element:hover + .tooltip-element"?

I'm sure there are some other caveats that would reveal themselves later even if this one is avoidable...