It looks like Jeff fixed this two months ago.
http://cvs.openacs.org/cvs/openacs-4/packages/acs-mail-lite/tcl/acs-mail-lite-procs.xql
The problem was that I upgraded using the openacs-5-1-compat tag instead of oacs-5-1. My understanding was packages should be tagged with openacs-5-1-compat, but it appears that's not always the case.
It appears the functionality is busted on the openacs-5-1-compat 'branch', so I'm going to tag it, since the changes seem very minor. I'm also going to bump the package to 0.21a, so that existing installs can upgrade via the package repository.