We don't store (findable) hostnames in the file, so don't search for them.
Don't search file if the host is listed in the spamblock_exempt.cfg file.
If you have a really large spamblock.cfg file, this could result in the
unnecessary reading of potential many megabytes of data for each inbound
SMTP connection. We might want to read this file even less frequently and
store the list in a searchable str_list_t or something.