Most X programs (xterm, firefox) remember the last highlighted text in the X selection buffer, including when the user clicks and deselects the highlighted portion. In Claws Mail, highlighted text is saved to the X selection buffer, but deselecting text (by clicking to reposition the cursor, for example) clears the X selection, so that middle clicking does nothing. It would be nice if Claws behaved as most other X programs.
I can confirm this issue. It's been a major annoyance for me since I started using Claws Maail.