excuse my english Rated 4 out of 5 stars

At http://majkl.bluemonk.org/tb you can DOWNLOAD the xpi.
But you can copy whole path http://majkl.bluemonk.org/tb/mailredirect-0.7.4-tb3-20100708.xpi to the "Filename" field of add-on install dialog (Thunderbird - Tools - Addons - Extensions - Install) - TB will download it directly. (Matt's URL works too when installing by this way.)

This review is for a previous version of the add-on (0.7.4).  This user has a previous review of this add-on.