the tolower function call, and possibly others, return an empty value when compiled with the -O2 option. My problem was fixed by following the advise found at the URL provided.
Steps to Reproduce:
1. emerge mail-filter/maildrop (with -O2 on amd64)
I can reproduce in my up-to-date gentoo hardened server (amd64).
CFLAGS="-O1" CXXFLAGS="-O1" emerge -1 maildrop
Please reopen if this is still a problem with newer versions. Thanks.