mirrors://gentoo/squirrelmail-20070519_0000-SVN.devel.tar.bz2 mirrors is not valid; also there's no ChangeLog entry for this version at all.
This should be fixed in the tree now. Jeremy: how many time did you test this before commiting it?
It has KEYWORDS=-* for a reason (Yes, I should've used package.mask!). I put it in portage to test as a possible solution to security bug #177820 and removed it in favor of patching 1.5.1. This ebuild is not in portage any more.