There is an explanation on Lasman's site, just before the drop_solo router:
http://www.nobaloney.net/downloads/spamblocker/DirectAdminSpamBlocker3/exim.conf.3.2.6a-RC
# if we have an alias, but no passwd entry we have to drop the email because the
# first alias is unseen (so that you can forward as well as save it)
# The save part is "seen" (virtual_user), but the forward before it isn't. This
# will be the spot where we "see" the email so that it doesn't send a bounce if
# we have an alias but no pop.
drop_solo_alias:
...
