IUSE.invalid 1 mail-client/smtpclient/smtpclient-1.0.1.ebuild: fullheaders You did not add the flag to use.local.desc. Also, it doesn't look like the flag does anything as is. I fixed two other small issues with the ebuild, you don't need to DEPEND on virtual/libc, and the header was screwed up in your new ebuild.
fixed, thanks.